Somnolence is a state of near-sleep
Sleep is a naturally recurring state characterized by reduced or absent consciousness, 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, and is more easily reversible than...

, a strong desire for sleep, or sleeping for unusually long periods (cf. hypersomnia
Hypersomnia is a disorder characterized by excessive amounts of sleepiness.There are two main categories of hypersomnia: primary hypersomnia and recurrent hypersomnia...

). 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
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...

. Somnolence goes back to the Latin "somnus" meaning "sleep."


Sleepiness can be dangerous when performing tasks that require constant concentration, such as driving a vehicle
A vehicle is a device that is designed or used to transport people or cargo. Most often vehicles are manufactured, such as bicycles, cars, motorcycles, trains, ships, boats, and aircraft....

. When a person is sufficiently fatigued, he or she may experience 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...



The human body can become sleepy in response to infection
An infection is the colonization of a host organism by parasite species. Infecting parasites seek to use the host's resources to reproduce, often resulting in disease...

. Such somnolence is one of several sickness behavior
Sickness behavior
thumb|350px|right|[[Michael Peter Ancher|Ancher, Michael]], "The Sick Girl", 1882, [[Statens Museum for Kunst]]Sickness behavior is a coordinated set of adaptive behavioral changes that develop in ill individuals during the course of an infection....

s or reactions to infection that some theorize evolved
Evolutionary medicine
Evolutionary medicine or Darwinian medicine is the application of modern evolutionary theory to understanding health and disease. It provides a complementary scientific approach to the present mechanistic explanations that dominate medical science, and particularly modern medical education...

 to promote recovery by conserving energy while the body fights the infection using fever
Fever is a common medical sign characterized by an elevation of temperature above the normal range of due to an increase in the body temperature regulatory set-point. This increase in set-point triggers increased muscle tone and shivering.As a person's temperature increases, there is, in...

 and other means.

Associated conditions

  • advanced sleep phase syndrome
    Advanced sleep phase syndrome
    Advanced sleep phase syndrome , also known as the advanced sleep-phase type of circadian rhythm sleep disorder, is a condition in which patients feel very sleepy and go to bed early in the evening and wake up very early in the morning Advanced sleep phase syndrome (ASPS), also known as the...

  • Alice in Wonderland syndrome
    Alice in Wonderland syndrome
    Alice-in-Wonderland syndrome , also known as Todd's syndrome, is a disorienting neurological condition that affects human perception. Sufferers may experience micropsia, macropsia, or size distortion of other sensory modalities...

  • brain edema
    Cerebral edema
    Cerebral edema or cerebral œdema is an excess accumulation of water in the intracellular or extracellular spaces of the brain.-Vasogenic:Due to a breakdown of tight endothelial junctions which make up the blood-brain barrier...

  • cerebral hypoxia
    Cerebral hypoxia
    Cerebral hypoxia refers to a reduced supply of oxygen to the brain. Cerebral anoxia refers to a complete lack of oxygen to the brain. There are four separate categories of cerebral hypoxia; in order of severity they are; diffuse cerebral hypoxia , focal cerebral ischemia, cerebral infarction, and...

  • Chronic Mental Fatigue Syndrome
  • clinical depression
    Clinical depression
    Major depressive disorder is a mental disorder characterized by an all-encompassing low mood accompanied by low self-esteem, and by loss of interest or pleasure in normally enjoyable activities...

    , especially seasonal affective disorder
    Seasonal affective disorder
    Seasonal affective disorder , also known as winter depression, winter blues, summer depression, summer blues, or seasonal depression, is a mood disorder in which people who have normal mental health throughout most of the year experience depressive symptoms in the winter or summer, spring or autumn...

  • delayed sleep phase syndrome
    Delayed sleep phase syndrome
    Delayed sleep-phase syndrome , also known as delayed sleep-phase disorder or delayed sleep-phase type , is a circadian rhythm sleep disorder, a chronic disorder of the timing of sleep, peak period of alertness, the core body temperature rhythm, hormonal and other daily rhythms, compared to the...

  • diabetes – ketoacidosis as example, but not balanced diabetes mellitus
  • encephalitis
    Encephalitis is an acute inflammation of the brain. Encephalitis with meningitis is known as meningoencephalitis. Symptoms include headache, fever, confusion, drowsiness, and fatigue...

     – (viral, bacterial or other agents)
  • epilepsy
    Epilepsy is a common chronic neurological disorder characterized by seizures. These seizures are transient signs and/or symptoms of abnormal, excessive or hypersynchronous neuronal activity in the brain.About 50 million people worldwide have epilepsy, and nearly two out of every three new cases...

     – after seizure
  • hydrocephalus
    Hydrocephalus , also known as "water in the brain," is a medical condition in which there is an abnormal accumulation of cerebrospinal fluid in the ventricles, or cavities, of the brain. This may cause increased intracranial pressure inside the skull and progressive enlargement of the head,...

  • hyperparathyroidism
    Hyperparathyroidism is overactivity of the parathyroid glands resulting in excess production of parathyroid hormone . The parathyroid hormone regulates calcium and phosphate levels and helps to maintain these levels...

  • 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...

  • hypothyroidism
    Hypothyroidism is a condition in which the thyroid gland does not make enough thyroid hormone.Iodine deficiency is the most common cause of hypothyroidism worldwide but it can be caused by other causes such as several conditions of the thyroid gland or, less commonly, the pituitary gland or...

  • infectious mononucleosis
    Infectious mononucleosis
    Infectious mononucleosis is an infectious, widespread viral...

     (glandular fever)
  • intracranial hemorrhage
    Intracranial hemorrhage
    An intracranial hemorrhage is a hemorrhage, or bleeding, within the skull.-Causes:Intracranial bleeding occurs when a blood vessel within the skull is ruptured or leaks. It can result from physical trauma or nontraumatic causes such as a ruptured aneurysm...

     such as due to ruptured aneurysm
    An aneurysm or aneurism is a localized, blood-filled balloon-like bulge in the wall of a blood vessel. Aneurysms can commonly occur in arteries at the base of the brain and an aortic aneurysm occurs in the main artery carrying blood from the left ventricle of the heart...

  • increased intracranial pressure
    Intracranial pressure
    Intracranial pressure is the pressure inside the skull and thus in the brain tissue and cerebrospinal fluid . The body has various mechanisms by which it keeps the ICP stable, with CSF pressures varying by about 1 mmHg in normal adults through shifts in production and absorption of CSF...

    ; for example, due to brain tumors
  • Lyme disease
    Lyme disease
    Lyme disease, or Lyme borreliosis, is an emerging infectious disease caused by at least three species of bacteria belonging to the genus Borrelia. Borrelia burgdorferi sensu stricto is the main cause of Lyme disease in the United States, whereas Borrelia afzelii and Borrelia garinii cause most...

  • medications
    • analgesics – mostly prescribed or illicit opiates such as OxyContin or heroin
    • anticonvulsants / antiepileptics – such as phenytoin (Dilantin), carbamazepine (Tegretol), lamotrigine
      Lamotrigine, marketed in the US and most of Europe as Lamictal by GlaxoSmithKline, is an anticonvulsant drug used in the treatment of epilepsy and bipolar disorder. It is also used as an adjunct in treating depression, though this is considered off-label usage...

       (Lamictal), Lyrica (pregbalin), Gabapentin
      Gabapentin is a pharmaceutical drug, specifically a GABA analogue. It was originally developed for the treatment of epilepsy, and currently is also used to relieve neuropathic pain...

    • antidepressants – for instance, sertraline
      Sertraline hydrochloride is an antidepressant of the selective serotonin reuptake inhibitor class. It was introduced to the market by Pfizer in 1991. Sertraline is primarily used to treat major depression in adult outpatients as well as obsessive–compulsive, panic, and social anxiety disorders in...

      , venlafaxine
      Venlafaxine is an antidepressant of the serotonin-norepinephrine reuptake inhibitor class. First introduced by Wyeth in 1993, now marketed by Pfizer, it is licensed for the treatment of major depressive disorder , as a treatment for generalized anxiety disorder, and comorbid indications in...

       and fluoxetine
      Fluoxetine is an antidepressant of the selective serotonin reuptake inhibitor class. It is manufactured and marketed by Eli Lilly and Company...

    • antihistamines – for instance, 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 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...

    • antipsychotics – for example, thioridazine
      Thioridazine is a piperidine typical antipsychotic drug belonging to the phenothiazine drug group and was previously widely used in the treatment of schizophrenia and psychosis...

      , quetiapine
      Quetiapine , is an atypical antipsychotic approved for the treatment of schizophrenia, and bipolar disorder....

      , and olanzapine
      Olanzapine is an atypical antipsychotic, approved by the FDA for the treatment of schizophrenia and bipolar disorder...

       (Zyprexa) and haloperidol
      Haloperidol is a typical antipsychotic. It is in the butyrophenone class of antipsychotic medications and has pharmacological effects similar to the phenothiazines....

       and Geodon.
    • dopamine agonists used in the treatment of Parkinson's disease
      Parkinson's disease
      Parkinson's disease is a degenerative disorder of the central nervous system...

       – e.g. pergolide and ropinirole and Mirapex, used to treat restless leg syndrome.
    • HIV
      Human immunodeficiency virus is a lentivirus that causes acquired immunodeficiency syndrome , a condition in humans in which progressive failure of the immune system allows life-threatening opportunistic infections and cancers to thrive...

       medications – for example, Sustiva and medications containing efavirenz
      Efavirenz is a non-nucleoside reverse transcriptase inhibitor and is used as part of highly active antiretroviral therapy for the treatment of a human immunodeficiency virus type 1....

    • Hormone replacement therapy (male-to-female)
      Hormone replacement therapy (male-to-female)
      Hormone replacement therapy for transgender and transsexual people changes the balance of sex hormones in their bodies. Some intersex people also receive HRT, either starting in childhood to confirm the sex to which they were assigned, or later, if this assignment has proven to be incorrect...

    • 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...

       medications – such as Norvasc
    • tranquilizers / hypnotics – especially benzodiazepines, such as temazepam
      Temazepam is an intermediate-acting 3-hydroxy benzodiazepine. It is mostly prescribed for the short-term treatment of sleeplessness in patients who have difficulty maintaining sleep...

       (Restoril) or nitrazepam
      Nitrazepam is a type of benzodiazepine drug and is marketed in English-speaking countries under the following brand names: Alodorm, Arem, Insoma, Mogadon, Nitrados, Nitrazadon, Ormodon, Paxadorm, Remnos, and Somnite...

       (Mogadon), and 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, such as amobarbital
      Amobarbital is a drug that is a barbiturate derivative. It has sedative-hypnotic and analgesic properties. It is a white crystalline powder with no odor and a slightly bitter taste. It was first synthesized in Germany in 1923...

       (Amytal) or secobarbital
      Secobarbital sodium is a barbiturate derivative drug that was first synthesized in 1928 in Germany. It possesses anaesthetic, anticonvulsant, sedative and hypnotic properties...

    • other agents impacting the central nervous system
      Central nervous system
      The central nervous system is the part of the nervous system that integrates the information that it receives from, and coordinates the activity of, all parts of the bodies of bilaterian animals—that is, all multicellular animals except sponges and radially symmetric animals such as jellyfish...

      , in sufficient or toxic doses
  • 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...

  • sickness behavior
    Sickness behavior
    thumb|350px|right|[[Michael Peter Ancher|Ancher, Michael]], "The Sick Girl", 1882, [[Statens Museum for Kunst]]Sickness behavior is a coordinated set of adaptive behavioral changes that develop in ill individuals during the course of an infection....

  • 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...

  • 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...

     / 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:...

  • starvation
    Starvation is a severe deficiency in caloric energy, nutrient and vitamin intake. It is the most extreme form of malnutrition. In humans, prolonged starvation can cause permanent organ damage and eventually, death...

  • stroke
    A stroke, previously known medically as a cerebrovascular accident , is the rapidly developing loss of brain function due to disturbance in the blood supply to the brain. This can be due to ischemia caused by blockage , or a hemorrhage...

  • traumatic brain injury
    Traumatic brain injury
    Traumatic brain injury , also known as intracranial injury, occurs when an external force traumatically injures the brain. TBI can be classified based on severity, mechanism , or other features...

  • trypanosomiasis
    Trypanosomiasis or trypanosomosis is the name of several diseases in vertebrates caused by parasitic protozoan trypanosomes of the genus Trypanosoma. Approximately 500,000 men, women and children in 36 countries of sub-Saharan Africa suffer from human African trypanosomiasis which is caused by...

See also

  • Chronic Fatigue Syndrome
    Chronic fatigue syndrome
    Chronic fatigue syndrome is the most common name used to designate a significantly debilitating medical disorder or group of disorders generally defined by persistent fatigue accompanied by other specific symptoms for a minimum of six months, not due to ongoing exertion, not substantially...

  • Decision fatigue
    Decision fatigue
    In decision making and psychology, decision fatigue refers to the deteriorating quality of decisions made by an individual, after a long session of decision making. It is now understood as one of the causes of irrational trade-offs in decision making. For instance, judges in court have been shown...

  • 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:...

  • Hypersomnia
    Hypersomnia is a disorder characterized by excessive amounts of sleepiness.There are two main categories of hypersomnia: primary hypersomnia and recurrent hypersomnia...

  • Dyssomnia
    Dyssomnias are a broad classification of sleeping disorders that make it difficult to get to sleep, or to remain sleeping.Dyssomnias are primary disorders of initiating or maintaining sleep or of excessive sleepiness and are characterized by a disturbance in the amount, quality, or timing of...

  • Fatigue (physical)
    Fatigue (physical)
    Fatigue is a state of awareness describing a range of afflictions, usually associated with physical and/or mental weakness, though varying from a general state of lethargy to a specific work-induced burning sensation within one's muscles...

  • Postprandial somnolence
  • Restless legs syndrome
    Restless legs syndrome
    Restless legs syndrome or Willis-Ekbom disease is a neurological disorder characterized by an irresistible urge to move one's body to stop uncomfortable or odd sensations. It most commonly affects the legs, but can affect the arms, torso, and even phantom limbs...

  • Periodic limb movement
  • Later stages of Hepatic Encephalopathy
  • The Drowsy Chaperone
    The Drowsy Chaperone
    The Drowsy Chaperone is a musical with book by Bob Martin and Don McKellar and music and lyrics by Lisa Lambert and Greg Morrison. It debuted in 1998 at The Rivoli in Toronto and opened on Broadway on 1 May 2006. The show won the Tony Award for Best Book and Best Score. It started as a spoof of old...

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.