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 being in hibernation or a coma. Sleep is 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 decreases metabolism only about 5–10%. Hibernating animals need to sleep despite the hypometabolism seen in hibernation, and must return from hypothermia to euthermia in order to sleep, making sleeping "energetically expensive."

Read more about Sleep:  Siesta or Nap, Sleep Debt, Genetics, Functions, Dreaming, Insomnia, Obstructive Sleep Apnea, Other Sleep Disorders, Anthropology of Sleep, Sleep in Non-humans

Other articles related to "sleep":

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, and sleep deprivation is followed by longer or deeper rebound sleep ...
Go To Sleep (disambiguation)
... "Go to Sleep" is a song by Radiohead ... Go to Sleep or Go 2 Sleep may also refer to "Go to Sleep" (Lupe Fiasco song) "Go to Sleep", a song by Barbra Streisand from the film On a Clear Day You Can See Forever "Go to Sleep", a song ...
Fiction (The Comsat Angels Album)
... Fiction was less gloomy than the Comsats' previous album, Sleep No More ... "I certainly didn't want to make another record as intense as Sleep No More — at least not immediately ... Sleep No More was so dark that I felt it skewed things a bit — possibly even mentally for me ...
Dorsal Raphe Nucleus - Role in Narcolepsy
... narcolepsy, this is logical, as the raphe nuclei have been known to play a role in the sleep/wake cycle ... of what normally happens during REM sleep, when all muscle tone is lost except for the eyes ... during waking hours, partially active during non-REM sleep, and have almost ceased during REM sleep ...
May 2005 In Science - May 1, 2005
... to wake up its owner only when he has emerged from deep sleep ... The clock, called SleepSmart, measures the sleep cycle and waits until its owner is in his lightest phase of sleep before rousing him ...

Famous quotes containing the word sleep:

    You can’t stay married in a situation where you are afraid to go to sleep in case your wife might cut your throat.
    Mike Tyson (b. 1966)

    How gladly would I meet,
    Mortality, my sentence, and be earth
    Insensible! how glad would lay me down,
    As in my mother’s lap! There I should rest,
    And sleep secure.
    John Milton (1608–1674)

    So they lived. They didn’t sleep together, but they had children.
    —Russian saying popular in the Soviet period, trans. by Vladimir Ivanovich Shlyakov (1993)