Sleep
Overview
Sleep is a naturally recurring state characterized by reduced or absent consciousness
Consciousness
Consciousness is a term that refers to the relationship between the mind and the world with which it interacts. It has been defined as: subjectivity, awareness, the ability to experience or to feel, wakefulness, having a sense of selfhood, and the executive control system of the mind...

, relatively suspended sensory activity, and inactivity of nearly all voluntary muscles. It is distinguished from quiet wakefulness by a decreased ability to react to stimuli
Stimulus (physiology)
In physiology, a stimulus is a detectable change in the internal or external environment. The ability of an organism or organ to respond to external stimuli is called sensitivity....

, and is more easily reversible than being in hibernation
Hibernation
Hibernation is a state of inactivity and metabolic depression in animals, characterized by lower body temperature, slower breathing, and lower metabolic rate. Hibernating animals conserve food, especially during winter when food supplies are limited, tapping energy reserves, body fat, at a slow rate...

 or a coma
Coma
In medicine, a coma is a state of unconsciousness, lasting more than 6 hours in which a person cannot be awakened, fails to respond normally to painful stimuli, light or sound, lacks a normal sleep-wake cycle and does not initiate voluntary actions. A person in a state of coma is described as...

. Sleep is also a heightened anabolic state, accentuating the growth and rejuvenation of the immune, nervous, skeletal and muscular systems. It is observed in all mammals, all birds, and many reptiles, amphibians, and fish.

The purposes and mechanisms of sleep are only partially clear and are the subject of intense research.
Quotations

Sleep has no favorites.

Anonymous

There is a time for many words, and there is also a time for sleep.

Homer

Even where sleep is concerned, too much is a bad thing.'

Homer

I have never taken any exercise except sleeping and resting.

Mark Twain

All men, whilst they are awake, are in one common world: but each of them, when he is asleep, is in a world of his own.

Plutarch

Laugh and the world laughs with you, snore and you sleep alone.

Anthony Burgess

Most people are walking in their sleep; turn them around, start them in the opposite direction, and they wouldn’t even know the difference.

Al Hubbard

There are two types of people in this world, good and bad. The good sleep better, but the bad seem to enjoy the waking hours much more.

Woody Allen

Death, so called, is a thing which makes men weep: And yet a third of Life is passed in sleep.

Lord Byron

"I love sleep. My life has the tendency to fall apart when I'm awake, you know?"

Ernest Hemingway

Encyclopedia
Sleep is a naturally recurring state characterized by reduced or absent consciousness
Consciousness
Consciousness is a term that refers to the relationship between the mind and the world with which it interacts. It has been defined as: subjectivity, awareness, the ability to experience or to feel, wakefulness, having a sense of selfhood, and the executive control system of the mind...

, relatively suspended sensory activity, and inactivity of nearly all voluntary muscles. It is distinguished from quiet wakefulness by a decreased ability to react to stimuli
Stimulus (physiology)
In physiology, a stimulus is a detectable change in the internal or external environment. The ability of an organism or organ to respond to external stimuli is called sensitivity....

, and is more easily reversible than being in hibernation
Hibernation
Hibernation is a state of inactivity and metabolic depression in animals, characterized by lower body temperature, slower breathing, and lower metabolic rate. Hibernating animals conserve food, especially during winter when food supplies are limited, tapping energy reserves, body fat, at a slow rate...

 or a coma
Coma
In medicine, a coma is a state of unconsciousness, lasting more than 6 hours in which a person cannot be awakened, fails to respond normally to painful stimuli, light or sound, lacks a normal sleep-wake cycle and does not initiate voluntary actions. A person in a state of coma is described as...

. Sleep is also a heightened anabolic state, accentuating the growth and rejuvenation of the immune, nervous, skeletal and muscular systems. It is observed in all mammals, all birds, and many reptiles, amphibians, and fish.

The purposes and mechanisms of sleep are only partially clear and are the subject of intense research. Sleep is often thought to help conserve energy, but actually decreases metabolism only about 5–10%. Hibernating animals need to sleep despite the hypometabolism seen in hibernation, and in fact they must return from hypothermia
Hypothermia
Hypothermia is a condition in which core temperature drops below the required temperature for normal metabolism and body functions which is defined as . Body temperature is usually maintained near a constant level of through biologic homeostasis or thermoregulation...

 to euthermy in order to sleep, making sleeping "energetically expensive."

Sleep stages

In mammals and birds, sleep is divided into two broad types: rapid eye movement (REM) and non-rapid eye movement (NREM or non-REM) sleep. Each type has a distinct set of associated physiological, neurological, and psychological features. The American Academy of Sleep Medicine
American Academy of Sleep Medicine
The American Academy of Sleep Medicine is a United States professional society for the medical subspecialty of sleep medicine. It was established in 1975. The AASM is the only professional society that is dedicated exclusively to the medical subspecialty of sleep medicine...

 (AASM) further divides NREM into three stages: N1, N2, and N3, the last of which is also called delta
Delta wave
A delta wave is a high amplitude brain wave with a frequency of oscillation between 0–4 hertz. Delta waves, like other brain waves, are recorded with an electroencephalogram and are usually associated with the deepest stages of sleep , also known as slow-wave sleep , and aid in characterizing the...

 sleep or slow-wave sleep
Slow-wave sleep
Slow-wave sleep , often referred to as deep sleep, consists of stages 3 and 4 of non-rapid eye movement sleep, according to the Rechtschaffen & Kales standard of 1968. As of 2008, the American Academy of Sleep Medicine has discontinued the use of stage 4, such that the previous stages 3 and 4 now...

 (SWS).
Sleep proceeds in cycles of REM and NREM, the order normally being N1 → N2 → N3 → N2 → REM. There is a greater amount of deep sleep (stage N3) earlier in the sleep cycle, while the proportion of REM sleep increases later in the sleep cycle and just before natural awakening.

The stages of sleep were first described in 1937 by Alfred Lee Loomis
Alfred Lee Loomis
Alfred Lee Loomis was an American attorney, investment banker, philanthropist, scientist/physicist, pioneer in military radar usages, inventor of the LORAN or Long Range Navigation System, and lifelong patron of scientific research...

 and his coworkers, who separated the different electroencephalography
Electroencephalography
Electroencephalography is the recording of electrical activity along the scalp. EEG measures voltage fluctuations resulting from ionic current flows within the neurons of the brain...

 (EEG) features of sleep into five levels (A to E), which represented the spectrum from wakefulness to deep sleep. In 1953, REM sleep was discovered as distinct, and thus William Dement and Nathaniel Kleitman
Nathaniel Kleitman
Nathaniel Kleitman was Professor Emeritus in Physiology at the University of Chicago. Author of the seminal 1939 book Sleep and Wakefulness, he is recognized as the father of American sleep research...

 reclassified sleep into four NREM stages and REM. The staging criteria were standardized in 1968 by Allan Rechtschaffen
Allan Rechtschaffen
Allan Rechtschaffen is a noted pioneer in the field of sleep research whose work includes some of the first laboratory studies of insomnia, narcolepsy, sleep apnea,and napping....

 and Anthony Kales in the "R&K sleep scoring manual." In the R&K standard, NREM sleep was divided into four stages, with slow-wave sleep comprising stages 3 and 4. In stage 3, delta waves made up less than 50% of the total wave patterns, while they made up more than 50% in stage 4. Furthermore, REM sleep was sometimes referred to as stage 5.

In 2004, the AASM commissioned the AASM Visual Scoring Task Force to review the R&K scoring system. The review resulted in several changes, the most significant being the combination of stages 3 and 4 into Stage N3. The revised scoring was published in 2007 as The AASM Manual for the Scoring of Sleep and Associated Events. Arousals and respiratory, cardiac, and movement events were also added.

Sleep stages and other characteristics of sleep are commonly assessed by polysomnography
Polysomnography
Polysomnography , also known as a sleep study, is a multi-parametric test used in the study of sleep and as a diagnostic tool in sleep medicine. The test result is called a polysomnogram, also abbreviated PSG...

 in a specialized sleep laboratory. Measurements taken include EEG of brain waves, electrooculography
Electrooculography
Electrooculography is a technique for measuring the resting potential of the retina. The resulting signal is called the electrooculogram. The main applications are in ophthalmological diagnosis and in recording eye movements...

 (EOG) of eye movements, and electromyography
Electromyography
Electromyography is a technique for evaluating and recording the electrical activity produced by skeletal muscles. EMG is performed using an instrument called an electromyograph, to produce a record called an electromyogram. An electromyograph detects the electrical potential generated by muscle...

 (EMG) of skeletal muscle
Skeletal muscle
Skeletal muscle is a form of striated muscle tissue existing under control of the somatic nervous system- i.e. it is voluntarily controlled. It is one of three major muscle types, the others being cardiac and smooth muscle...

 activity. In humans, each sleep cycle lasts from 90 to 110 minutes on average, and each stage may have a distinct physiological function. This can result in sleep that exhibits loss of consciousness
Unconsciousness
Unconsciousness is the condition of being not conscious—in a mental state that involves complete or near-complete lack of responsiveness to people and other environmental stimuli. Being in a comatose state or coma is a type of unconsciousness. Fainting due to a drop in blood pressure and a...

 but does not fulfill its physiological functions (i.e., one may still feel tired after apparently sufficient sleep).

Scientific studies on sleep have shown that sleep stage at awakening is an important factor in amplifying sleep inertia
Sleep inertia
Sleep inertia is a physiological state characterised by a decline in motor dexterity and a subjective feeling of grogginess immediately following an abrupt awakening. The impaired alertness may interfere with the ability to perform mental or physical tasks...

. Alarm clock
Alarm clock
An alarm clock is a clock that is designed to make a loud sound at a specific time. The primary use of these clocks is to awaken people from their night's sleep or short naps; they are sometimes used for other reminders as well. To stop the sound, a button or handle on the clock is pressed; but...

s involving sleep stage monitoring appeared on the market in 2005. Using sensing technologies such as EEG
EEG
EEG commonly refers to electroencephalography, a measurement of the electrical activity of the brain.EEG may also refer to:* Emperor Entertainment Group, a Hong Kong-based entertainment company...

 electrodes or accelerometers, these alarm clocks are supposed to wake people only from light sleep.

NREM sleep

According to the 2007 AASM standards, NREM consists of three stages. There is relatively little dreaming in NREM.

Stage N1 refers to the transition of the brain from alpha waves
Alpha Waves
Alpha Waves is an early 3D game that combines labyrinthine exploration with platform gameplay. By most definitions of the genre it could be considered to be the first 3D platform game, released in 1990, 6 years before the genre's seminal classic Super Mario 64...

 having a frequency of 8–13 Hz
Hertz
The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....

 (common in the awake state) to theta waves having a frequency of 4–7 Hz. This stage is sometimes referred to as somnolence or drowsy sleep. Sudden twitches and hypnic jerk
Hypnic jerk
A hypnic jerk, hypnagogic jerk, sleep start, or night start, is an involuntary myoclonic twitch which occurs during hypnagogia, just as a person is beginning to fall asleep, often causing them to awaken suddenly. Physically, hypnic jerks resemble the "jump" experienced by a person when...

s, also known as positive myoclonus
Myoclonus
Myoclonus is brief, involuntary twitching of a muscle or a group of muscles. It describes a medical sign and, generally, is not a diagnosis of a disease. Brief twitches are perfectly normal. The myoclonic twitches are usually caused by sudden muscle contractions; they also can result from brief...

, may be associated with the onset of sleep during N1. Some people may also experience hypnagogic hallucinations during this stage. During N1, the subject loses some muscle tone
Muscle tone
In physiology, medicine, and anatomy, muscle tone is the continuous and passive partial contraction of the muscles, or the muscle’s resistance to passive stretch during resting state. It helps maintain posture, and it declines during REM sleep.-Purpose:Unconscious nerve impulses maintain the...

 and most conscious awareness of the external environment. To put it in simpler form, you are in a very shallow sleep, can jerk awake easily, are slightly aware of your surroundings, aren't dreaming, and have no rapid eye movement.

Stage N2 is characterized by sleep spindle
Sleep spindle
A sleep spindle is a burst of brain activity visible on an EEG that occurs during stage 2 sleep. It consists of 12–14 Hz waves that occur for at least 0.5 seconds.-Function:...

s ranging from 11 to 16 Hz (most commonly 12–14 Hz) and K-complex
K-complex
A K-complex is an electroencephalography waveform that occurs during stage 2 of NREM sleep. It is the "largest event in healthy human EEG". They are more frequent in the first sleep cycles of sleep.K-complexes have two proposed functions...

es. During this stage, muscular activity as measured by EMG decreases, and conscious awareness of the external environment disappears. This stage occupies 45–55% of total sleep in adults.

Stage N3 (deep or slow-wave sleep
Slow-wave sleep
Slow-wave sleep , often referred to as deep sleep, consists of stages 3 and 4 of non-rapid eye movement sleep, according to the Rechtschaffen & Kales standard of 1968. As of 2008, the American Academy of Sleep Medicine has discontinued the use of stage 4, such that the previous stages 3 and 4 now...

) is characterized by the presence of a minimum of 20% delta wave
Delta wave
A delta wave is a high amplitude brain wave with a frequency of oscillation between 0–4 hertz. Delta waves, like other brain waves, are recorded with an electroencephalogram and are usually associated with the deepest stages of sleep , also known as slow-wave sleep , and aid in characterizing the...

s ranging from 0.5–2 Hz and having a peak-to-peak amplitude >75 μV. (EEG standards define delta waves to be from 0 to 4 Hz, but sleep standards in both the original R&K, as well as the new 2007 AASM guidelines have a range of 0.5–2 Hz.) This is the stage in which parasomnia
Parasomnia
For the 2008 horror film, see Parasomnia Parasomnias are a category of sleep disorders that involve abnormal and unnatural movements, behaviors, emotions, perceptions, and dreams that occur while falling asleep, sleeping, between sleep stages, or during arousal from sleep...

s such as night terror
Night terror
A night terror, also known as a sleep terror, incubus attack, or pavor nocturnus, is a parasomnia disorder that predominantly affects children, causing feelings of terror or dread, typically occur in the first few hours of sleep during stage 3 or 4 NREM sleep...

s, nocturnal enuresis, sleepwalking
Sleepwalking
Sleepwalking, also known as somnambulism, is a sleep disorder belonging to the parasomnia family. Sleepwalkers arise from the slow wave sleep stage in a state of low consciousness and perform activities that are usually performed during a state of full consciousness...

, and somniloquy
Somniloquy
Somniloquy or sleep-talking is a parasomnia that refers to talking aloud while sleep. It can be quite loud, ranging from simple sounds to long speeches, and can occur many times during sleep...

 occur. Many illustrations and descriptions still show a stage N3 with 20–50% delta waves and a stage N4 with greater than 50% delta waves; these have been combined as stage N3.

REM sleep

Rapid eye movement sleep, or REM sleep, accounts for 20–25% of total sleep time in most human adults. The criteria for REM sleep include rapid eye movements as well as a rapid low-voltage EEG. Most memorable dreaming occurs in this stage. At least in mammals, a descending muscular atonia is seen. Such paralysis may be necessary to protect organisms from self-damage through physically acting out scenes from the often-vivid dreams that occur during this stage.

Timing

Sleep timing is controlled by the circadian clock, sleep-wake homeostasis
Homeostasis
Homeostasis is the property of a system that regulates its internal environment and tends to maintain a stable, constant condition of properties like temperature or pH...

, and in humans, within certain bounds, willed behavior. The circadian clock—an inner timekeeping, temperature-fluctuating, enzyme-controlling device—works in tandem with adenosine, a neurotransmitter that inhibits many of the bodily processes associated with wakefulness. Adenosine is created over the course of the day; high levels of adenosine lead to sleepiness. In diurnal animals, sleepiness occurs as the circadian element causes the release of the hormone melatonin
Melatonin
Melatonin , also known chemically as N-acetyl-5-methoxytryptamine, is a naturally occurring compound found in animals, plants, and microbes...

 and a gradual decrease in core body temperature
Thermoregulation
Thermoregulation is the ability of an organism to keep its body temperature within certain boundaries, even when the surrounding temperature is very different...

. The timing is affected by one's chronotype
Chronotype
Chronotype is an attribute of animals, including human beings, reflecting at what time of the day their physical functions are active, change or reach a certain level...

. It is the circadian rhythm that determines the ideal timing of a correctly structured and restorative sleep episode.

Homeostatic sleep propensity (the need for sleep as a function of the amount of time elapsed since the last adequate sleep episode) must be balanced against the circadian element for satisfactory sleep. Along with corresponding messages from the circadian clock, this tells the body it needs to sleep. Sleep offset (awakening) is primarily determined by circadian rhythm. A person who regularly awakens at an early hour will generally not be able to sleep much later than his or her normal waking time, even if moderately sleep-deprived.

Sleep duration is affected by the gene DEC2. Some people have a mutation of this gene; they sleep two hours less than normal. Neurology professor Ying-Hui Fu and her colleagues bred mice that carried the DEC2 mutation and slept less than normal mice.

Adult

The optimal amount of sleep is not a meaningful concept unless the timing of that sleep is seen in relation to an individual's circadian rhythm
Circadian rhythm
A circadian rhythm, popularly referred to as body clock, is an endogenously driven , roughly 24-hour cycle in biochemical, physiological, or behavioural processes. Circadian rhythms have been widely observed in plants, animals, fungi and cyanobacteria...

s. A person's major sleep episode is relatively inefficient and inadequate when it occurs at the "wrong" time of day; one should be asleep at least six hours before the lowest body temperature. The timing is correct when the following two circadian markers occur after the middle of the sleep episode and before awakening:
  • maximum concentration of the hormone melatonin, and
  • minimum core body temperature.


For more information on the human circadian rhythm and body temperature, see Biological markers (in the article Circadian rhythm
Circadian rhythm
A circadian rhythm, popularly referred to as body clock, is an endogenously driven , roughly 24-hour cycle in biochemical, physiological, or behavioural processes. Circadian rhythms have been widely observed in plants, animals, fungi and cyanobacteria...

).

Human sleep needs can vary by age and among individuals, and sleep is considered to be adequate when there is no daytime sleepiness or dysfunction. Moreover, self-reported sleep duration is only moderately correlated with actual sleep time as measured by actigraphy
Actigraphy
Actigraphy is a non-invasive method of monitoring human rest/activity cycles. A small actigraph unit, also called an actimetry sensor, is worn by a patient to measure gross motor activity. Motor activity often under test is that of the wrist, measured by an actigraph in a wrist-watch-like package....

, and those affected with sleep state misperception
Sleep state misperception
Sleep state misperception is a term in the International Classification of Sleep Disorders most commonly used for people who mistakenly perceive their sleep as wakefulness, though it has been proposed that it be applied to those who severely overestimate their sleep time as well...

 may typically report having slept only four hours despite having slept a full eight hours.

A University of California, San Diego
University of California, San Diego
The University of California, San Diego, commonly known as UCSD or UC San Diego, is a public research university located in the La Jolla neighborhood of San Diego, California, United States...

 psychiatry study of more than one million adults found that people who live the longest self-report sleeping for six to seven hours each night. Another study of sleep duration and mortality risk in women showed similar results. Other studies show that "sleeping more than 7 to 8 hours per day has been consistently associated with increased mortality," though this study suggests the cause is probably other factors such as depression and socioeconomic status, which would correlate statistically. It has been suggested that the correlation between lower sleep hours and reduced morbidity only occurs with those who wake after less sleep naturally, rather than those who use an alarm.

Researchers at the University of Warwick
University of Warwick
The University of Warwick is a public research university located in Coventry, United Kingdom...

 and University College London
University College London
University College London is a public research university located in London, United Kingdom and the oldest and largest constituent college of the federal University of London...

 have found that lack of sleep can more than double the risk of death from cardiovascular disease
Cardiovascular disease
Heart disease or cardiovascular disease are the class of diseases that involve the heart or blood vessels . While the term technically refers to any disease that affects the cardiovascular system , it is usually used to refer to those related to atherosclerosis...

, but that too much sleep can also be associated with a doubling of the risk of death, though not primarily from cardiovascular disease. Professor Francesco Cappuccio said, "Short sleep has been shown to be a risk factor
Risk factor
In epidemiology, a risk factor is a variable associated with an increased risk of disease or infection. Sometimes, determinant is also used, being a variable associated with either increased or decreased risk.-Correlation vs causation:...

 for weight gain, hypertension
Hypertension
Hypertension or high blood pressure is a cardiac chronic medical condition in which the systemic arterial blood pressure is elevated. What that means is that the heart is having to work harder than it should to pump the blood around the body. Blood pressure involves two measurements, systolic and...

, and Type 2 diabetes
Diabetes mellitus type 2
Diabetes mellitus type 2formerly non-insulin-dependent diabetes mellitus or adult-onset diabetesis a metabolic disorder that is characterized by high blood glucose in the context of insulin resistance and relative insulin deficiency. Diabetes is often initially managed by increasing exercise and...

, sometimes leading to mortality; but in contrast to the short sleep-mortality association, it appears that no potential mechanisms by which long sleep could be associated with increased mortality have yet been investigated. Some candidate causes for this include depression, low socioeconomic status, and cancer-related fatigue
Cancer-related fatigue
Cancer-related fatigue is a subjective symptom of fatigue that is experienced by nearly all cancer patients.Among patients receiving cancer treatment other than surgery, it is essentially universal. Fatigue is a normal and expected side effect from most forms of chemotherapy, radiation therapy,...

... In terms of prevention, our findings indicate that consistently sleeping around seven hours per night is optimal for health, and a sustained reduction may predispose to ill health."

Furthermore, sleep difficulties are closely associated with psychiatric disorders such as depression, alcoholism
Alcoholism
Alcoholism is a broad term for problems with alcohol, and is generally used to mean compulsive and uncontrolled consumption of alcoholic beverages, usually to the detriment of the drinker's health, personal relationships, and social standing...

, and bipolar disorder
Bipolar disorder
Bipolar disorder or bipolar affective disorder, historically known as manic–depressive disorder, is a psychiatric diagnosis that describes a category of mood disorders defined by the presence of one or more episodes of abnormally elevated energy levels, cognition, and mood with or without one or...

. Up to 90% of adults with depression are found to have sleep difficulties. Dysregulation found on EEG includes disturbances in sleep continuity, decreased delta sleep and altered REM patterns with regard to latency, distribution across the night and density of eye movements.

Hours by age

Children need more sleep per day in order to develop and function properly: up to 18 hours for newborn babies, with a declining rate as a child ages. A newborn baby spends almost 9 hours a day in REM sleep. By the age of five or so, only slightly over two hours is spent in REM. Studies say that school age children need about 10 to 11 hours of sleep.
Age and condition Average amount of sleep per day
Newborn up to 18 hours
1–12 months 14–18 hours
1–3 years 12–15 hours
3–5 years 11–13 hours
5–12 years 9–11 hours
Adolescents 9–10 hours
Adults, including elderly 7–8 hours
Pregnant women 8(+) hours

Sleep debt

Sleep debt is the effect of not getting enough rest and sleep; a large debt causes mental, emotional and physical fatigue.

Sleep debt results in diminished abilities to perform high-level cognitive functions. Neurophysiological and functional imaging studies have demonstrated that frontal regions of the brain are particularly responsive to homeostatic sleep pressure.

Scientists do not agree on how much sleep debt it is possible to accumulate; whether it is accumulated against an individual's average sleep or some other benchmark; nor on whether the prevalence of sleep debt among adults has changed appreciably in the industrialized world
Developed country
A developed country is a country that has a high level of development according to some criteria. Which criteria, and which countries are classified as being developed, is a contentious issue...

 in recent decades. It is likely that children are sleeping less than previously in Western societies
Western world
The Western world, also known as the West and the Occident , is a term referring to the countries of Western Europe , the countries of the Americas, as well all countries of Northern and Central Europe, Australia and New Zealand...

.

Genetics

It is suspected that a considerable amount of sleep-related behavior, such as when and how long a person needs to sleep, is regulated by our genetics. Researchers have discovered some evidence that seems to support this assumption.

Functions

The multiple theories proposed to explain the function of sleep reflect the as-yet incomplete understanding of the subject. (When asked, after 50 years of research, what he knew about the reason people sleep William Dement, founder of Stanford University
Stanford University
The Leland Stanford Junior University, commonly referred to as Stanford University or Stanford, is a private research university on an campus located near Palo Alto, California. It is situated in the northwestern Santa Clara Valley on the San Francisco Peninsula, approximately northwest of San...

's Sleep Research Center, answered, "As far as I know, the only reason we need to sleep that is really, really solid is because we get sleepy.") It is likely that sleep evolved to fulfill some primeval function and took on multiple functions over time (analogous to the larynx
Larynx
The larynx , commonly called the voice box, is an organ in the neck of amphibians, reptiles and mammals involved in breathing, sound production, and protecting the trachea against food aspiration. It manipulates pitch and volume...

, which controls the passage of food and air, but descended over time to develop speech cabilities).

If sleep were not essential, one would expect to find:
  • Animal species that do not sleep at all
  • Animals that do not need recovery sleep when they stay awake longer than usual
  • Animals that suffer no serious consequences as a result of lack of sleep


Outside of a few basal
Basal (phylogenetics)
In phylogenetics, a basal clade is the earliest clade to branch in a larger clade; it appears at the base of a cladogram.A basal group forms an outgroup to the rest of the clade, such as in the following example:...

 animals that have no brain or a very simple one, no animals have been found to date that satisfy any of these criteria. While some varieties of shark, such as great whites and hammerheads, must remain in motion at all times to move oxygenated water over their gills, it is possible they still sleep one cerebral hemisphere at a time as marine mammals do. However it remains to be shown definitively whether any fish is capable of unihemispheric sleep.

Some of the many proposed functions of sleep are as follows:

Restoration

Wound healing
Wound healing
Wound healing, or cicatrisation, is an intricate process in which the skin repairs itself after injury. In normal skin, the epidermis and dermis exists in a steady-state equilibrium, forming a protective barrier against the external environment...

 has been shown to be affected by sleep. A study conducted by Gumustekin et al. in 2004 shows sleep deprivation
Sleep deprivation
Sleep deprivation is the condition of not having enough sleep; it can be either chronic or acute. A chronic sleep-restricted state can cause fatigue, daytime sleepiness, clumsiness and weight loss or weight gain. It adversely affects the brain and cognitive function. Few studies have compared the...

 hindering the healing
Healing
Physiological healing is the restoration of damaged living tissue, organs and biological system to normal function. It is the process by which the cells in the body regenerate and repair to reduce the size of a damaged or necrotic area....

 of burns on rats.

It has been shown that sleep deprivation affects the immune system
Immune system
An immune system is a system of biological structures and processes within an organism that protects against disease by identifying and killing pathogens and tumor cells. It detects a wide variety of agents, from viruses to parasitic worms, and needs to distinguish them from the organism's own...

. In a study by Zager et al. in 2007, rats were deprived of sleep for 24 hours. When compared with a control group, the sleep-deprived rats' blood test
Blood test
A blood test is a laboratory analysis performed on a blood sample that is usually extracted from a vein in the arm using a needle, or via fingerprick....

s indicated a 20% decrease in white blood cell
White blood cell
White blood cells, or leukocytes , are cells of the immune system involved in defending the body against both infectious disease and foreign materials. Five different and diverse types of leukocytes exist, but they are all produced and derived from a multipotent cell in the bone marrow known as a...

 count, a significant change in the immune system. It is now possible to state that "sleep loss impairs immune function and immune challenge alters sleep," and it has been suggested that mammalian species which invest in longer sleep times are investing in the immune system, as species with the longer sleep times have higher white blood cell counts.

It has yet to be proven that sleep duration affects somatic
Somatic
The term somatic means 'of the body',, relating to the body. In medicine, somatic illness is bodily, not mental, illness. The term is often used in biology to refer to the cells of the body in contrast to the germ line cells which usually give rise to the gametes...

 growth. One study by Jenni et al. in 2007 recorded growth, height, and weight, as correlated to parent-reported time in bed in 305 children over a period of nine years (age 1–10). It was found that "the variation of sleep duration among children does not seem to have an effect on growth." It has been shown that sleep—more specifically, slow-wave sleep (SWS)—does affect growth hormone
Growth hormone
Growth hormone is a peptide hormone that stimulates growth, cell reproduction and regeneration in humans and other animals. Growth hormone is a 191-amino acid, single-chain polypeptide that is synthesized, stored, and secreted by the somatotroph cells within the lateral wings of the anterior...

 levels in adult men. During eight hours' sleep, Van Cauter, Leproult, and Plat found that the men with a high percentage of SWS (average 24%) also had high growth hormone secretion, while subjects with a low percentage of SWS (average 9%) had low growth hormone secretion.

There are multiple arguments supporting the restorative function of sleep. The metabolic phase during sleep is anabolic; anabolic hormones such as growth hormones (as mentioned above) are secreted preferentially during sleep. The duration of sleep among species is, in general, inversely related
Inverse relationship
An inverse or negative relationship is a mathematical relationship in which one variable, say y, decreases as another, say x, increases. For a linear relation, this can be expressed as y = a-bx, where -b is a constant value less than zero and a is a constant...

 to animal size and directly related to basal metabolic rate
Basal metabolic rate
Basal Metabolic Rate , and the closely related resting metabolic rate , is the amount of daily energy expended by humans and other animals at rest. Rest is defined as existing in a neutrally temperate environment while in the post-absorptive state...

. Rats with a very high basal metabolic rate sleep for up to 14 hours a day, whereas elephants and giraffes with lower BMRs sleep only 3–4 hours per day.

Energy conservation could as well have been accomplished by resting quiescent without shutting off the organism from the environment, potentially a dangerous situation. A sedentary nonsleeping animal is more likely to survive predators, while still preserving energy. Sleep, therefore, seems to serve another purpose, or other purposes, than simply conserving energy; for example, hibernating animals waking up from hibernation go into rebound sleep because of lack of sleep during the hibernation period. They are definitely well-rested and are conserving energy during hibernation, but need sleep for something else. Rats kept awake indefinitely develop skin lesions, hyperphagia, loss of body mass, hypothermia
Hypothermia
Hypothermia is a condition in which core temperature drops below the required temperature for normal metabolism and body functions which is defined as . Body temperature is usually maintained near a constant level of through biologic homeostasis or thermoregulation...

, and, eventually, fatal sepsis
Sepsis
Sepsis is a potentially deadly medical condition that is characterized by a whole-body inflammatory state and the presence of a known or suspected infection. The body may develop this inflammatory response by the immune system to microbes in the blood, urine, lungs, skin, or other tissues...

.

Ontogenesis

According to the ontogenetic hypothesis of REM sleep, the activity occurring during neonatal REM sleep (or active sleep) seems to be particularly important to the developing organism (Marks et al., 1995). Studies investigating the effects of deprivation of active sleep have shown that deprivation early in life can result in behavioral problems, permanent sleep disruption, decreased brain mass (Mirmiran et al., 1983), and an abnormal amount of neuronal cell death.

REM sleep appears to be important for development of the brain. REM sleep occupies the majority of time of sleep of infants, who spend most of their time sleeping. Among different species, the more immature the baby is born, the more time it spends in REM sleep. Proponents also suggest that REM-induced muscle inhibition in the presence of brain activation exists to allow for brain development by activating the synapses, yet without any motor consequences that may get the infant in trouble. Additionally, REM deprivation results in developmental abnormalities later in life.

However, this does not explain why older adults still need REM sleep. Aquatic mammal
Marine mammal
Marine mammals, which include seals, whales, dolphins, and walruses, form a diverse group of 128 species that rely on the ocean for their existence. They do not represent a distinct biological grouping, but rather are unified by their reliance on the marine environment for feeding. The level of...

 infants do not have REM sleep in infancy; REM sleep in those animals increases as they age.

Memory processing

Scientists have shown numerous ways in which sleep is related to memory
Memory
In psychology, memory is an organism's ability to store, retain, and recall information and experiences. Traditional studies of memory began in the fields of philosophy, including techniques of artificially enhancing memory....

. In a study conducted by Turner, Drummond, Salamat, and Brown, working memory
Working memory
Working memory has been defined as the system which actively holds information in the mind to do verbal and nonverbal tasks such as reasoning and comprehension, and to make it available for further information processing...

 was shown to be affected by sleep deprivation. Working memory is important because it keeps information active for further processing and supports higher-level cognitive functions
Cognitive functions
In some forms of psychological testing, particularly those related to the Myers-Briggs Type Indicator, the cognitive functions are defined as different ways of perceiving and judging the world...

 such as decision making
Decision making
Decision making can be regarded as the mental processes resulting in the selection of a course of action among several alternative scenarios. Every decision making process produces a final choice. The output can be an action or an opinion of choice.- Overview :Human performance in decision terms...

, reasoning, and episodic memory
Episodic memory
Episodic memory is the memory of autobiographical events that can be explicitly stated. Semantic and episodic memory together make up the category of declarative memory, which is one of the two major divisions in memory...

. The study allowed 18 women and 22 men to sleep only 26 minutes per night over a four-day period. Subjects were given initial cognitive tests while well-rested, and then were tested again twice a day during the four days of sleep deprivation. On the final test, the average working memory span of the sleep-deprived group had dropped by 38% in comparison to the control group.

Memory seems to be affected differently by certain stages of sleep such as REM and slow-wave sleep
Slow-wave sleep
Slow-wave sleep , often referred to as deep sleep, consists of stages 3 and 4 of non-rapid eye movement sleep, according to the Rechtschaffen & Kales standard of 1968. As of 2008, the American Academy of Sleep Medicine has discontinued the use of stage 4, such that the previous stages 3 and 4 now...

 (SWS). In one study, multiple groups of human subjects were used: wake control groups and sleep test groups. Sleep and wake groups were taught a task and were then tested on it, both on early and late nights, with the order of nights balanced across participants. When the subjects' brains were scanned during sleep, hypnograms revealed that SWS was the dominant sleep stage during the early night, representing around 23% on average for sleep stage activity. The early-night test group performed 16% better on the declarative memory
Declarative memory
Declarative memory is one of two types of long term human memory. It refers to memories which can be consciously recalled such as facts and knowledge. Its counterpart is known as non-declarative or Procedural memory, which refers to unconscious memories such as skills...

 test than the control group. During late-night sleep, REM became the most active sleep stage at about 24%, and the late-night test group performed 25% better on the procedural memory
Procedural memory
Procedural memory is memory for how to do things. Procedural memory guides the processes we perform and most frequently resides below the level of conscious awareness. When needed, procedural memories are automatically retrieved and utilized for the execution of the integrated procedures involved...

 test than the control group. This indicates that procedural memory benefits from late, REM-rich sleep, whereas declarative memory benefits from early, SWS-rich sleep.

A study conducted by Datta indirectly supports these results. The subjects chosen were 22 male rats. A box was constructed wherein a single rat could move freely from one end to the other. The bottom of the box was made of a steel grate. A light would shine in the box accompanied by a sound. After a five-second delay, an electrical shock would be applied. Once the shock commenced, the rat could move to the other end of the box, ending the shock immediately. The rat could also use the five-second delay to move to the other end of the box and avoid the shock entirely. The length of the shock never exceeded five seconds. This was repeated 30 times for half the rats. The other half, the control group, was placed in the same trial, but the rats were shocked regardless of their reaction. After each of the training sessions, the rat would be placed in a recording cage for six hours of polygraphic recordings. This process was repeated for three consecutive days. This study found that during the posttrial sleep recording session, rats spent 25.47% more time in REM sleep after learning trials than after control trials. These trials support the results of the Born et al. study, indicating an obvious correlation between REM sleep and procedural knowledge
Procedural knowledge
Procedural knowledge, also known as imperative knowledge, is the knowledge exercised in the performance of some task. See below for the specific meaning of this term in cognitive psychology and intellectual property law....

.

An observation of the Datta study is that the learning group spent 180% more time in SWS than did the control group during the post-trial sleep-recording session. This phenomenon is supported by a study performed by Kudrimoti, Barnes, and McNaughton. This study shows that after spatial exploration activity, patterns of hippocampal place cells are reactivated during SWS following the experiment. In a study by Kudrimoti et al., seven rats were run through a linear track using rewards on either end. The rats would then be placed in the track for 30 minutes to allow them to adjust (PRE), then they ran the track with reward-based training for 30 minutes (RUN), and then they were allowed to rest for 30 minutes. During each of these three periods, EEG
EEG
EEG commonly refers to electroencephalography, a measurement of the electrical activity of the brain.EEG may also refer to:* Emperor Entertainment Group, a Hong Kong-based entertainment company...

 data were collected for information on the rats' sleep stages. Kudrimoti et al. computed the mean firing rates of hippocampal place cells during prebehavior SWS (PRE) and three ten-minute intervals in postbehavior SWS (POST) by averaging across 22 track-running sessions from seven rats. The results showed that ten minutes after the trial RUN session, there was a 12% increase in the mean firing rate of hippocampal place cells from the PRE level; however, after 20 minutes, the mean firing rate returned rapidly toward the PRE level. The elevated firing of hippocampal place cells during SWS after spatial exploration could explain why there were elevated levels of SWS sleep in Datta's study, as it also dealt with a form of spatial exploration.

A study has also been done involving direct current stimulation
Transcranial direct current stimulation
Transcranial direct current stimulation is a form of neurostimulation which uses constant, low current delivered directly to the brain area of interest via small electrodes...

 to the prefrontal cortex
Prefrontal cortex
The prefrontal cortex is the anterior part of the frontal lobes of the brain, lying in front of the motor and premotor areas.This brain region has been implicated in planning complex cognitive behaviors, personality expression, decision making and moderating correct social behavior...

 to increase the amount of slow oscillations during SWSfe. The direct current stimulation greatly enhanced word-pair retention the following day, giving evidence that SWS plays a large role in the consolidation of episodic memories.

The different studies all suggest that there is a correlation between sleep and the complex functions of memory. Harvard sleep researchers Saper and Stickgold
Robert Stickgold
Robert Stickgold is an Associate Professor of Psychiatry at Harvard Medical School and Beth Israel Deaconess Medical Center. A preeminent sleep researcher, Dr. Stickgold has dedicated his life to understanding the relationship between sleep and learning. He is also a very active educator...

 point out that an essential part of memory and learning consists of nerve cell dendrite
Dendrite
Dendrites are the branched projections of a neuron that act to conduct the electrochemical stimulation received from other neural cells to the cell body, or soma, of the neuron from which the dendrites project...

s' sending of information to the cell body to be organized into new neuronal connections. This process demands that no external information is presented to these dendrites, and it is suggested that this may be why it is during sleep that memories and knowledge are solidified and organized.

Preservation

The "Preservation and Protection" theory holds that sleep serves an adaptive function. It protects the animal during that portion of the 24-hour day in which being awake, and hence roaming around, would place the individual at greatest risk. Organisms do not require 24 hours to feed themselves and meet other necessities. From this perspective of adaptation, organisms are safer by staying out of harm's way, where potentially they could be prey to other, stronger organisms. They sleep at times that maximize their safety, given their physical capacities and their habitats.

This theory fails to explain why the brain disengages from the external environment during normal sleep. However, the brain consumes a large proportion of the body's calories at any one time and preservation of energy could only occur by limiting its sensory inputs. Another argument against the theory is that sleep is not simply a passive consequence of removing the animal from the environment, but is a "drive"; animals alter their behaviors in order to obtain sleep. Therefore, circadian regulation is more than sufficient to explain periods of activity and quiescence that are adaptive to an organism, but the more peculiar specializations of sleep probably serve different and unknown functions. Moreover, the preservation theory needs to explain why carnivores like lions, which are on top of the food chain
Food chain
A food web depicts feeding connections in an ecological community. Ecologists can broadly lump all life forms into one of two categories called trophic levels: 1) the autotrophs, and 2) the heterotrophs...

 and thus have little to fear, sleep the most. It has been suggested that they need to minimize energy expenditure when not hunting.

Preservation also does not explain why aquatic mammals sleep while moving. Quiescence during these vulnerable hours would do the same and would be more advantageous, because the animal would still be able to respond to environmental challenges like predators, etc. Sleep rebound that occurs after a sleepless night will be maladaptive, but obviously must occur for a reason. A zebra falling asleep the day after it spent the sleeping time running from a lion is more, not less, vulnerable to predation.

Dreaming

Dreaming is the perceived experience of sensory images and sounds during sleep, in a sequence which the dreamer usually perceives more as an apparent participant than as an observer. Dreaming is stimulated by the pons
Pons
The pons is a structure located on the brain stem, named after the Latin word for "bridge" or the 16th-century Italian anatomist and surgeon Costanzo Varolio . It is superior to the medulla oblongata, inferior to the midbrain, and ventral to the cerebellum. In humans and other bipeds this means it...

 and mostly occurs during the REM phase of sleep.

People have proposed many hypotheses about the functions of dreaming. Sigmund Freud
Sigmund Freud
Sigmund Freud , born Sigismund Schlomo Freud , was an Austrian neurologist who founded the discipline of psychoanalysis...

 postulated that dreams are the symbolic expression of frustrated desires that had been relegated to the unconscious mind
Unconscious mind
The unconscious mind is a term coined by the 18th century German romantic philosopher Friedrich Schelling and later introduced into English by the poet and essayist Samuel Taylor Coleridge...

, and he used dream interpretation
Dream interpretation
Dream interpretation is the process of assigning meaning to dreams. In many ancient societies, such as those of Egypt and Greece, dreaming was considered a supernatural communication or a means of divine intervention, whose message could be unravelled by people with certain powers...

 in the form of psychoanalysis
Psychoanalysis
Psychoanalysis is a psychological theory developed in the late 19th and early 20th centuries by Austrian neurologist Sigmund Freud. Psychoanalysis has expanded, been criticized and developed in different directions, mostly by some of Freud's former students, such as Alfred Adler and Carl Gustav...

 to uncover these desires. See Freud: The Interpretation of Dreams
The Interpretation of Dreams
The Interpretation of Dreams is a book by psychoanalyst Sigmund Freud. The first edition begins:.The book introduces Freud's theory of the unconscious with respect to dream interpretation...

.

Freud's work concerns the psychological role of dreams, which does not exclude any physiological role they may have. Recent research claims that sleep has the overall role of consolidation and organization of synaptic connections formed during learning and experience. As such, Freud's work is not ruled out. Nevertheless, Freud's research has been expanded on, especially with regard to the organization and consolidation of recent memory
Memory
In psychology, memory is an organism's ability to store, retain, and recall information and experiences. Traditional studies of memory began in the fields of philosophy, including techniques of artificially enhancing memory....

.

Certain processes in the cerebral cortex have been studied by John Allan Hobson
Allan Hobson
John Allan Hobson, M.D. is an American psychiatrist and dream researcher.He is known for his research on Rapid eye movement sleep. He is Professor of Psychiatry, Emeritus, Harvard Medical School,...

 and Robert McCarley
Robert McCarley
Robert W. McCarley, MD, is Chair and Professor of Psychiatry at Harvard Medical School and the VA Boston Healthcare System. He is also Director of the Laboratory of Neuroscience located at the Brockton VA Medical Center and the McLean Hospital...

. In their activation synthesis theory, for example, they propose that dreams are caused by the random firing of neurons in the cerebral cortex
Cerebral cortex
The cerebral cortex is a sheet of neural tissue that is outermost to the cerebrum of the mammalian brain. It plays a key role in memory, attention, perceptual awareness, thought, language, and consciousness. It is constituted of up to six horizontal layers, each of which has a different...

 during the REM period. Neatly, this theory helps explain the irrationality of the mind during REM periods, as, according to this theory, the forebrain then creates a story
Narrative
A narrative is a constructive format that describes a sequence of non-fictional or fictional events. The word derives from the Latin verb narrare, "to recount", and is related to the adjective gnarus, "knowing" or "skilled"...

 in an attempt to reconcile and make sense of the nonsensical sensory information presented to it. Ergo, the odd nature of many dreams.

Hypnotics

  • Nonbenzodiazepine
    Nonbenzodiazepine
    The nonbenzodiazepines, also called benzodiazepine-like drugs, are a class of psychoactive drugs pharmacologically resembling the benzodiazepines, with similar benefits, side effects and risks, despite having dissimilar or entirely different chemical structures.-Classes:There are currently three...

     hypnotics such as eszopiclone
    Eszopiclone
    Eszopiclone, marketed by Sepracor under the brand-name Lunesta, is a nonbenzodiazepine hypnotic used as a treatment for insomnia. Eszopiclone is the active dextrorotatory stereoisomer of zopiclone, and belongs to the class of drugs known as cyclopyrrolones.Eszopiclone is a short acting...

     (Lunesta), zaleplon
    Zaleplon
    Zaleplon is a sedative/hypnotic, mainly used for insomnia. It is a nonbenzodiazepine hypnotic from the pyrazolopyrimidine class. In terms of adverse effects zaleplon appears to offer little improvement compared to both benzodiazepines and other non-benzodiazepine Z-drugs.Sonata is manufactured by...

     (Sonata), and zolpidem
    Zolpidem
    Zolpidem is a prescription medication used for the short-term treatment of insomnia, as well as some brain disorders. It is a short-acting nonbenzodiazepine hypnotic of the imidazopyridine class that potentiates gamma-aminobutyric acid , an inhibitory neurotransmitter, by binding to GABAA...

     (Ambien) are commonly used as sleep aids prescribed by doctors to treat forms of insomnia. Nonbenzodiazepines are the most commonly prescribed and OTC
    OTC
    OTC may refer to:* Oakwood Technology College* Owatonna Tool Company* Oklahoma Tax Commission* Odenton Town Center* Officer in Tactical Command* Officer Training Corps* Offshore Technology Conference* Ohio Turnpike Commission...

     sleep aids used worldwide and have been greatly growing in use since the 1990s. They target the GABAA receptor.
  • Benzodiazepine
    Benzodiazepine
    A benzodiazepine is a psychoactive drug whose core chemical structure is the fusion of a benzene ring and a diazepine ring...

    s target the GABAA receptor also, and as such, they are commonly used sleep aids as well, though benzodiazepines have been found to decrease REM sleep.
  • Antihistamine
    Antihistamine
    An H1 antagonist is a histamine antagonist of the H1 receptor that serves to reduce or eliminate effects mediated by histamine, an endogenous chemical mediator released during allergic reactions...

    s, such as diphenhydramine
    Diphenhydramine
    Diphenhydramine hydrochloride is a first-generation antihistamine possessing anticholinergic, antitussive, antiemetic, and sedative properties which is mainly used to treat allergies. Like most other first-generation antihistamines, the drug also has a powerful hypnotic effect, and for this reason...

     (Benadryl) and doxylamine
    Doxylamine
    Doxylamine is one of the many sedating antihistamines used by itself as a short-term sedative, and in combination with other drugs as a night-time cold and allergy relief drug...

     (found in various OTC medicines, such as NyQuil
    NyQuil
    NyQuil is a brand of over the counter medication which is intended to relieve various symptoms of the common cold. Because all of the medications within the NyQuil imprint contain sedating antihistamines, hypnotics, and/or alcohol, they are intended to be taken before sleep...

    )
  • Alcohol
    Alcoholic beverage
    An alcoholic beverage is a drink containing ethanol, commonly known as alcohol. Alcoholic beverages are divided into three general classes: beers, wines, and spirits. They are legally consumed in most countries, and over 100 countries have laws regulating their production, sale, and consumption...

     – Often, people start drinking alcohol in order to get to sleep (alcohol is initially a sedative and will cause somnolence
    Somnolence
    Somnolence is a state of near-sleep, a strong desire for sleep, or sleeping for unusually long periods . It has two distinct meanings, referring both to the usual state preceding falling asleep, and the chronic condition referring to being in that state independent of a circadian rhythm...

    , encouraging sleep). However, being addicted to alcohol can lead to disrupted sleep, because alcohol has a rebound effect
    Rebound effect
    The rebound effect, or rebound phenomenon, is the tendency of some medications, when discontinued suddenly, to cause a return of the symptoms it relieved, and that, to a degree stronger than they were before treatment first began...

     later in the night. As a result, there is strong evidence linking alcoholism and forms of insomnia. Alcohol also reduces REM sleep.
  • Barbiturate
    Barbiturate
    Barbiturates are drugs that act as central nervous system depressants, and can therefore produce a wide spectrum of effects, from mild sedation to total anesthesia. They are also effective as anxiolytics, as hypnotics, and as anticonvulsants...

    s cause drowsiness and have actions similar to alcohol in that they have a rebound effect
    Rebound effect
    The rebound effect, or rebound phenomenon, is the tendency of some medications, when discontinued suddenly, to cause a return of the symptoms it relieved, and that, to a degree stronger than they were before treatment first began...

     and inhibit REM sleep, so they are not used as a long-term sleep aid.
  • Melatonin
    Melatonin
    Melatonin , also known chemically as N-acetyl-5-methoxytryptamine, is a naturally occurring compound found in animals, plants, and microbes...

     is a naturally occurring hormone that regulates sleepiness. It is made in the brain, where tryptophan is converted into serotonin and then into melatonin, which is released at night by the pineal gland
    Pineal gland
    The pineal gland is a small endocrine gland in the vertebrate brain. It produces the serotonin derivative melatonin, a hormone that affects the modulation of wake/sleep patterns and seasonal functions...

     to induce and maintain sleep. Melatonin supplementation may be used as a sleep aid, both as a hypnotic
    Hypnotic
    Hypnotic drugs are a class of psychoactives whose primary function is to induce sleep and to be used in the treatment of insomnia and in surgical anesthesia...

     and as a chronobiotic
    Chronobiotic
    Chronobiotic is a substance that is capable of therapeutically re-entering short - term dissociated or long - term desynchronized circadian rhythms, or prophylactically preventing their disruption following an environmental insult. In more simple words, chronobiotic is defined as the agent causes...

     (see phase response curve
    Phase response curve
    A phase response curve illustrates the transient change in the cycle period of an oscillation induced by a perturbation as a function of the phase at which it is received...

    , PRC).
  • Siesta
    Siesta
    A siesta is a short nap taken in the early afternoon, often after the midday meal. Such a period of sleep is a common tradition in some countries, particularly those where the weather is warm....

     and the "post-lunch dip" – Many people have a temporary drop in alertness in the early afternoon, commonly known as the "post-lunch dip." While a large meal can make a person feel sleepy, the post-lunch dip is mostly an effect of the biological clock
    Circadian rhythm
    A circadian rhythm, popularly referred to as body clock, is an endogenously driven , roughly 24-hour cycle in biochemical, physiological, or behavioural processes. Circadian rhythms have been widely observed in plants, animals, fungi and cyanobacteria...

    . People naturally feel most sleepy (have the greatest "drive for sleep") at two times of the day about 12 hours apart—for example, at 2:00 a.m. and 2:00 p.m. At those two times, the body clock "kicks in." At about 2 p.m. (14:00), it overrides the homeostatic buildup of sleep debt, allowing several more hours of wakefulness. At about 2 a.m. (02:00), with the daily sleep debt paid off, it "kicks in" again to ensure a few more hours of sleep.
  • Tryptophan
    Tryptophan
    Tryptophan is one of the 20 standard amino acids, as well as an essential amino acid in the human diet. It is encoded in the standard genetic code as the codon UGG...

     – The amino acid tryptophan is a building block of proteins. It has been claimed to contribute to sleepiness, since it is a precursor of the neurotransmitter serotonin, involved in sleep regulation. However, no solid data have ever linked modest dietary changes in tryptophan to changes in sleep.
  • Marijuana
    Cannabis (drug)
    Cannabis, also known as marijuana among many other names, refers to any number of preparations of the Cannabis plant intended for use as a psychoactive drug or for medicinal purposes. The English term marijuana comes from the Mexican Spanish word marihuana...

     – Some people use marijuana to induce sleepiness. Users often report relaxation and drowsiness.

Stimulants

  • Amphetamine
    Amphetamine
    Amphetamine or amfetamine is a psychostimulant drug of the phenethylamine class which produces increased wakefulness and focus in association with decreased fatigue and appetite.Brand names of medications that contain, or metabolize into, amphetamine include Adderall, Dexedrine, Dextrostat,...

     (dextroamphetamine
    Dextroamphetamine
    Dextroamphetamine is a psychostimulant drug which is known to produce increased wakefulness and focus as well as decreased fatigue and decreased appetite....

    , and a related, slightly more powerful drug methamphetamine
    Methamphetamine
    Methamphetamine is a psychostimulant of the phenethylamine and amphetamine class of psychoactive drugs...

    , etc.) are used to treat narcolepsy
    Narcolepsy
    Narcolepsy is a chronic sleep disorder, or dyssomnia, characterized by excessive sleepiness and sleep attacks at inappropriate times, such as while at work. People with narcolepsy often experience disturbed nocturnal sleep and an abnormal daytime sleep pattern, which often is confused with insomnia...

    . Their most common effects are anxiety, insomnia, stimulation, increased alertness, and decreased hunger.
  • Caffeine
    Caffeine
    Caffeine is a bitter, white crystalline xanthine alkaloid that acts as a stimulant drug. Caffeine is found in varying quantities in the seeds, leaves, and fruit of some plants, where it acts as a natural pesticide that paralyzes and kills certain insects feeding on the plants...

     is a stimulant
    Stimulant
    Stimulants are psychoactive drugs which induce temporary improvements in either mental or physical function or both. Examples of these kinds of effects may include enhanced alertness, wakefulness, and locomotion, among others...

     that works by slowing the action of the hormones in the brain that cause somnolence
    Somnolence
    Somnolence is a state of near-sleep, a strong desire for sleep, or sleeping for unusually long periods . It has two distinct meanings, referring both to the usual state preceding falling asleep, and the chronic condition referring to being in that state independent of a circadian rhythm...

    , particularly by acting as an antagonist
    Antagonist
    An antagonist is a character, group of characters, or institution, that represents the opposition against which the protagonist must contend...

     at adenosine receptors. Effective dosage is individual, in part dependent on prior usage. It can cause a rapid reduction in alertness as it wears off.
  • Cocaine
    Cocaine
    Cocaine is a crystalline tropane alkaloid that is obtained from the leaves of the coca plant. The name comes from "coca" in addition to the alkaloid suffix -ine, forming cocaine. It is a stimulant of the central nervous system, an appetite suppressant, and a topical anesthetic...

     and crack cocaine
    Crack cocaine
    Crack cocaine is the freebase form of cocaine that can be smoked. It may also be termed rock, hard, iron, cavvy, base, or just crack; it is the most addictive form of cocaine. Crack rocks offer a short but intense high to smokers...

     – Studies on cocaine have shown its effects to be mediated through the circadian rhythm system. This may be related to the onset of hypersomnia
    Hypersomnia
    Hypersomnia is a disorder characterized by excessive amounts of sleepiness.There are two main categories of hypersomnia: primary hypersomnia and recurrent hypersomnia...

     (oversleeping) in regard to "cocaine-induced sleep disorder."
  • MDMA, including similar drugs like MDA
    3,4-Methylenedioxyamphetamine
    3,4-Methylenedioxyamphetamine , also known as tenamfetamine , is a psychedelic and entactogenic drug of the phenethylamine and amphetamine chemical classes...

    , MMDA, or bk-MDMA
    Methylenedioxymethcathinone
    Methylone, also known as "M1", 3,4-methylenedioxy-N-methylcathinone, bk-MDMA, is an entactogen and stimulant of the phenethylamine, amphetamine, and cathinone classes. It was originally patented by Nick Wald and Adam Fantoma in 1996 as an antidepressant...

     – The class of drugs called empathogen-entactogen
    Empathogen-entactogen
    The terms empathogen and entactogen are used to describe a class of psychoactive drugs that produce distinctive emotional and social effects similar to those of MDMA. Putative members of this class include 2C-B, 2C-I, MDMA, MDA, MDEA, MBDB, 2C-T-7, and 2C-T-2, among others...

    s keep users awake with intense euphoria. Commonly known as "ecstasy."
  • Methylphenidate
    Methylphenidate
    Methylphenidate is a psychostimulant drug approved for treatment of attention-deficit hyperactivity disorder, postural orthostatic tachycardia syndrome and narcolepsy. It may also be prescribed for off-label use in treatment-resistant cases of lethargy, depression, neural insult and obesity...

     – Commonly known by the brand names Ritalin and Concerta, methylphenidate is similar in action to amphetamine and cocaine; its chemical composition more closely resembles that of cocaine.
  • Tobacco
    Tobacco
    Tobacco is an agricultural product processed from the leaves of plants in the genus Nicotiana. It can be consumed, used as a pesticide and, in the form of nicotine tartrate, used in some medicines...

     – Tobacco has been found not only to disrupt but also to reduce total sleep time. In studies, users have described more daytime drowsiness than nonsmokers.
  • Other analeptic
    Analeptic
    An analeptic, in medicine, is a central nervous system stimulant medication. The term analeptic may also refer specifically to a respiratory analeptic , a drug that acts on the central nervous system to stimulate the breathing muscles, improving respiration.Historically, the term has referred to...

     drugs like Modafinil
    Modafinil
    Modafinil is an analeptic drug manufactured by Cephalon, and is approved by the U.S. Food and Drug Administration for the treatment of narcolepsy, shift work sleep disorder, and excessive daytime sleepiness associated with obstructive sleep apnea...

     and Armodafinil
    Armodafinil
    Armodafinil is a stimulant-like drug produced by the pharmaceutical company Cephalon Inc., which was approved by the FDA on June 15, 2007...

     are prescribed to treat narcolepsy, hypersomnia
    Hypersomnia
    Hypersomnia is a disorder characterized by excessive amounts of sleepiness.There are two main categories of hypersomnia: primary hypersomnia and recurrent hypersomnia...

    , shift work sleep disorder
    Shift work sleep disorder
    Shift work sleep disorder is a circadian rhythm sleep disorder characterized by insomnia and excessive sleepiness affecting people whose work hours are scheduled during the typical sleep period...

    , and other conditions causing Excessive Daytime Sleepiness
    Excessive daytime sleepiness
    Excessive daytime sleepiness is characterized by persistent sleepiness, and often a general lack of energy, even after apparently adequate night time sleep...

    . The precise mechanism of these CNS stimulants is not known, but they have been shown to increase both the release of monoamines and levels of hypothalamic histamine, thereby promoting wakefulness.

Sleep problems

There are many reasons for poor sleep. For example, excessive exposure to bright light within hours of bedtime or simply resisting the urge to fall asleep can trigger a "second wind," which then can temporarily make it difficult to fall asleep afterwards. Following sleep hygienic principles may solve problems of physical or emotional discomfort. When the culprit is pain, illness, drugs, or stress, the cause must be treated. Sleep disorder
Sleep disorder
A sleep disorder, or somnipathy, is a medical disorder of the sleep patterns of a person or animal. Some sleep disorders are serious enough to interfere with normal physical, mental and emotional functioning...

s (including the sleep apnea
Sleep apnea
Sleep apnea is a sleep disorder characterized by abnormal pauses in breathing or instances of abnormally low breathing, during sleep. Each pause in breathing, called an apnea, can last from a few seconds to minutes, and may occur 5 to 30 times or more an hour. Similarly, each abnormally low...

s, narcolepsy
Narcolepsy
Narcolepsy is a chronic sleep disorder, or dyssomnia, characterized by excessive sleepiness and sleep attacks at inappropriate times, such as while at work. People with narcolepsy often experience disturbed nocturnal sleep and an abnormal daytime sleep pattern, which often is confused with insomnia...

, primary insomnia
Insomnia
Insomnia is most often defined by an individual's report of sleeping difficulties. While the term is sometimes used in sleep literature to describe a disorder demonstrated by polysomnographic evidence of disturbed sleep, insomnia is often defined as a positive response to either of two questions:...

, periodic limb movement disorder
Nocturnal myoclonus
Periodic limb movement disorder , previously known as nocturnal myoclonus, is a sleep disorder where the patient moves limbs involuntarily during sleep, and has symptoms or problems related to the movement....

 (PLMD), restless leg syndrome (RLS), and the circadian rhythm sleep disorder
Circadian rhythm sleep disorder
Circadian rhythm sleep disorders are a family of sleep disorders affecting, among other things, the timing of sleep. People with circadian rhythm sleep disorders are unable to sleep and wake at the times required for normal work, school, and social needs. They are generally able to get enough sleep...

s) are treatable. Fatal familial insomnia
Fatal familial insomnia
Fatal familial insomnia is a very rare autosomal dominant inherited prion disease of the brain. It is almost always caused by a mutation to the protein PrPC, but can also develop spontaneously in patients with a non-inherited mutation variant called sporadic fatal insomnia...

, or FFI, a genetic disease with no known treatment or cure, is characterized by increasing insomnia as one of its symptoms; ultimately sufferers of the disease stop sleeping entirely, before dying of the disease.

Older people
Old age
Old age consists of ages nearing or surpassing the average life span of human beings, and thus the end of the human life cycle...

 are more easily awakened by disturbances in the environment and may to some degree lose the ability to consolidate sleep.

Other treatments

There are various techniques and products which aim to improve sleep. These range from returning to sleep on the floor since beds are a relatively recent invention in human history, to various mattress designs.

A 2010 review of published scientific research suggested that exercise generally improves sleep for most people, and helps sleep disorders such as insomnia. The optimum time to exercise may be 4 to 8 hours before bedtime, though exercise at any time of day is beneficial, with the possible exception of heavy exercise taken shortly before bedtime, which may disturb sleep. However there is insufficient evidence to draw detailed conclusions about the relationship between exercise and sleep.

White noise
White noise
White noise is a random signal with a flat power spectral density. In other words, the signal contains equal power within a fixed bandwidth at any center frequency...

 appears to be a promising treatment for insomnia
Insomnia
Insomnia is most often defined by an individual's report of sleeping difficulties. While the term is sometimes used in sleep literature to describe a disorder demonstrated by polysomnographic evidence of disturbed sleep, insomnia is often defined as a positive response to either of two questions:...

.

Anthropology of sleep

Research suggests that sleep patterns vary significantly across cultures. The most striking differences are between societies that have plentiful sources of artificial light and ones that do not. The primary difference appears to be that pre-light cultures have more broken-up sleep patterns. For example, people might go to sleep far sooner after the sun sets, but then wake up several times throughout the night, punctuating their sleep with periods of wakefulness, perhaps lasting several hours. The boundaries between sleeping and waking are blurred in these societies. Some observers believe that nighttime sleep in these societies is most often split into two main periods, the first characterized primarily by deep sleep and the second by REM sleep.

Some societies display a fragmented sleep pattern in which people sleep at all times of the day and night for shorter periods. In many nomadic or hunter-gatherer
Hunter-gatherer
A hunter-gatherer or forage society is one in which most or all food is obtained from wild plants and animals, in contrast to agricultural societies which rely mainly on domesticated species. Hunting and gathering was the ancestral subsistence mode of Homo, and all modern humans were...

 societies, people will sleep on and off throughout the day or night depending on what is happening. Plentiful artificial light has been available in the industrialized West since at least the mid-19th century, and sleep patterns have changed significantly everywhere that lighting has been introduced. In general, people sleep in a more concentrated burst through the night, going to sleep much later, although this is not always true.

In some societies, people generally sleep with at least one other person (sometimes many) or with animals. In other cultures, people rarely sleep with anyone but a most intimate relation, such as a spouse. In almost all societies, sleeping partners are strongly regulated by social standards. For example, people might only sleep with their immediate family, extended family
Extended family
The term extended family has several distinct meanings. In modern Western cultures dominated by nuclear family constructs, it has come to be used generically to refer to grandparents, uncles, aunts, and cousins, whether they live together within the same household or not. However, it may also refer...

, spouses, their children, children of a certain age, children of specific gender, peers of a certain gender, friends, peers of equal social rank, or with no one at all. Sleep may be an actively social time, depending on the sleep groupings, with no constraints on noise or activity.

People sleep in a variety of locations. Some sleep directly on the ground; others on a skin or blanket; others sleep on platforms or bed
Bed
A bed is a large piece of furniture used as a place to sleep, relax, or engage in sexual relations.Most modern beds consist of a mattress on a bed frame, with the mattress resting either on a solid base, often wooden slats, or a sprung base...

s. Some sleep with blankets, some with pillows, some with simple headrests, some with no head support. These choices are shaped by a variety of factors, such as climate, protection from predators, housing type, technology, and the incidence of pests.

Sleep in non-humans


Neurological sleep states can be difficult to detect in some animals. In these cases, sleep may be defined using behavioral characteristics such as minimal movement, postures typical for the species, and reduced responsiveness to external stimulation. Sleep is quickly reversible, as opposed to hibernation or coma
Coma
In medicine, a coma is a state of unconsciousness, lasting more than 6 hours in which a person cannot be awakened, fails to respond normally to painful stimuli, light or sound, lacks a normal sleep-wake cycle and does not initiate voluntary actions. A person in a state of coma is described as...

, and sleep deprivation is followed by longer or deeper rebound sleep. Herbivores, who require a long waking period to gather and consume their diet, typically sleep less each day than similarly sized carnivores, who might well consume several days' supply of meat in a sitting.

Horses and other herbivorous ungulates can sleep while standing, but must necessarily lie down for REM sleep (which causes muscular atony) for short periods. Giraffes, for example, only need to lie down for REM sleep for a few minutes at a time. Bats sleep while hanging upside down. Some aquatic mammals and some birds can sleep with one half of the brain while the other half is awake, so-called unihemispheric slow-wave sleep
Unihemispheric slow-wave sleep
Unihemispheric slow-wave sleep , also termed asymmetric slow-wave sleep , is characterized by slow-wave activity in one hemisphere of the brain while a low voltage EEG, characteristic of wakefulness, is present in the other. The phenomenon has been observed in a number of terrestrial and aquatic as...

. Birds and mammals have cycles of non-REM and REM sleep (as described above for humans), though birds' cycles are much shorter and they do not lose muscle tone (go limp) to the extent that most mammals do.

Many mammals sleep for a large proportion of each 24-hour period when they are very young. However, killer whales and some dolphin
Dolphin
Dolphins are marine mammals that are closely related to whales and porpoises. There are almost forty species of dolphin in 17 genera. They vary in size from and , up to and . They are found worldwide, mostly in the shallower seas of the continental shelves, and are carnivores, mostly eating...

s do not sleep during the first month of life. Such differences may be explained by the ability of land-mammal newborns to be easily protected by parents while sleeping, while marine animals must, even while very young, be more continuously vigilant for predators.

See also

  • Cortisol awakening response
    Cortisol awakening response
    The cortisol awakening response is an increase of about 50% in cortisol levels occurring 20–30 minutes after awakening in the morning in some people. This rise is superimposed upon the late-night rise in cortisol which occurs before awakening...

  • Microsleep
    Microsleep
    A microsleep is an episode of sleep which may last for a fraction of a second or up to thirty seconds. Often, it is the result of sleep deprivation, mental fatigue, depression, sleep apnea, hypoxia, narcolepsy, or hypersomnia...

  • Morvan's syndrome
    Morvan's syndrome
    Morvan’s Syndrome, or Morvan’s fibrillary chorea , is a rare autoimmune disease named after nineteenth century French physician Augustin Marie Morvan...

  • National Sleep Foundation
    National Sleep Foundation
    The National Sleep Foundation is an independent nonprofit organization in the USA whose objectives are to improve public health and safety by achieving understanding of sleep and sleep disorders, and to support sleep-related education, research, and advocacy.Established in 1990, the NSF relies on...

  • Polyphasic sleep
    Polyphasic sleep
    Polyphasic sleep, a term coined by early 20th-century psychologist J.S. Szymanski, refers to the practice of sleeping multiple times in a 24-hour period—usually more than two, in contrast to biphasic sleep or monophasic sleep . It does not imply any particular sleep schedule...

  • Power nap
    Power nap
    A power nap is a short sleep which terminates before the occurrence of deep sleep or slow-wave sleep , intended to quickly revitalize the subject. The expression was coined by Cornell University social psychologist James Maas.-Characteristics:...

  • Sleep architecture
    Sleep architecture
    Sleep architecture describes the structure and pattern of sleep and encompasses several variables. Sleep quotas refer to the amount of time spent in REM and NREM sleep. Sleep duration is the total time spent asleep in a 24 hour period. The duration of a NREM-REM cycle is also an important aspect...

  • Sleep disorder
    Sleep disorder
    A sleep disorder, or somnipathy, is a medical disorder of the sleep patterns of a person or animal. Some sleep disorders are serious enough to interfere with normal physical, mental and emotional functioning...

  • Sleep medicine
    Sleep medicine
    Sleep medicine is a medical specialty or subspecialty devoted to the diagnosis and therapy of sleep disturbances and disorders. From the middle of the 20th century, research has provided increasing knowledge and answered many questions about sleep-wake functioning. The rapidly evolving field has...

  • Sudden infant death syndrome
    Sudden infant death syndrome
    Sudden infant death syndrome is marked by the sudden death of an infant that is unexpected by medical history, and remains unexplained after a thorough forensic autopsy and a detailed death scene investigation. An infant is at the highest risk for SIDS during sleep, which is why it is sometimes...

  • Sudden unexpected death syndrome

Positions, practices, and rituals

  • Sleeping position
    Sleeping position
    The sleeping position is the body configuration assumed by a person during or prior to sleeping.It has been shown to have health implications, particularly for babies...

  • Co-sleeping
    Co-sleeping
    Co-sleeping is a practice in which babies and young children sleep close to one or both parents, as opposed to in a separate room. It is standard practice in many parts of the world, and is practiced by a significant minority in countries where cribs are also used...

  • Hypnosis
    Hypnosis
    Hypnosis is "a trance state characterized by extreme suggestibility, relaxation and heightened imagination."It is a mental state or imaginative role-enactment . It is usually induced by a procedure known as a hypnotic induction, which is commonly composed of a long series of preliminary...

  • Meditation
    Meditation
    Meditation is any form of a family of practices in which practitioners train their minds or self-induce a mode of consciousness to realize some benefit....

  • Neutral spine
    Neutral spine
    A neutral spine or good posture refers to the "three natural curves [that] are present in a healthy spine."- Posture :The word "posture" comes from the Latin verb "ponere" which is defined as "to put or place." The general concept of human posture refers to "the carriage of the body as a whole, the...

  • Sleep hygiene
    Sleep hygiene
    Sleep hygiene can be defined as the controlling of "all behavioural and environmental factors that precede sleep and may interfere with sleep." It is the practice of following guidelines in an attempt to ensure more restful, effective sleep which can promote daytime alertness and help treat or...

  • Yoga-nidra
  • World Sleep Day
    World Sleep Day
    The World Sleep Day is an annual event organized by the World Sleep Day Committee of the World Association of Sleep Medicine since 2008...


External links

  • National Sleep Foundation, information on School Start Time and Sleep at National Sleep Foundation
    National Sleep Foundation
    The National Sleep Foundation is an independent nonprofit organization in the USA whose objectives are to improve public health and safety by achieving understanding of sleep and sleep disorders, and to support sleep-related education, research, and advocacy.Established in 1990, the NSF relies on...

  • HealthySleep.med.harvard.edu, from the Division of Sleep Medicine at Harvard Medical School
    Harvard Medical School
    Harvard Medical School is the graduate medical school of Harvard University. It is located in the Longwood Medical Area of the Mission Hill neighborhood of Boston, Massachusetts....

     and WGBH
    WGBH-TV
    WGBH-TV, channel 2, is a non-commercial educational public television station located in Boston, Massachusetts, USA. WGBH-TV is a member station of the Public Broadcasting Service , and produces more than two-thirds of PBS's national prime time television programming...

     Educational Foundation
  • PLOSjournals.org, Is Sleep Essential? by Chiara Cirelli and Giulio Tononi, from the Public Library of Science
    Public Library of Science
    The Public Library of Science is a nonprofit open-access scientific publishing project aimed at creating a library of open access journals and other scientific literature under an open content license...

    , Biology
  • NIH.gov, National Center on Sleep Disorders Research
The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
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