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

 
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