Sleeping

Sleep is a naturally recurring state of mind and body characterized by altered consciousness, relatively inhibited sensory activity, inhibition of nearly all voluntary muscles, and reduced interactions with surroundings. It is distinguished from wakefulness by a decreased ability to react to stimuli, but is more easily reversed than the state of hibernation or of being comatose. Mammalian sleep occurs in repeating periods, in which the body alternates between two highly distinct modes known as non-REM and REM sleep. REM stands for "rapid eye movement" but involves many other aspects including virtual paralysis of the body.

During sleep, most systems in an animal are in an anabolic state, and building up the immune, nervous, skeletal, and muscular systems. Sleep in non-human animals is observed in mammals, birds, reptiles, amphibians, and some fish, and, in some form, in insects and even in simpler animals such as nematodes. The internal circadian clock promotes sleep daily at night in diurnal organisms (such as humans) and in the day in nocturnal organisms (such as rodents). However, sleep patterns vary among individual humans and even more widely among other species. In the last century, artificial light has in many areas of the world substantially altered sleep timing among both humans and many other species.

The diverse purposes and mechanisms of sleep are the subject of substantial ongoing research. Sleep seems to assist animals with improvements in the body and mind. Recent research has shown that an essential function of sleep is to remove waste products from the brain.

A well-known feature of sleep in humans is the dream, an experience typically recounted in narrative form, which resembles waking life while in progress, but which usually can later be distinguished as fantasy. Sleep is sometimes confused with unconsciousness, but is quite different in terms of thought process.

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