Stages of sleep

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 being comatose. 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 are in an anabolic state, and building up the immune, nervous, skeletal, and muscular systems. The internal circadian clock promotes sleep daily at night. However, sleep patterns vary among individuals. In the last century, artificial light has in many areas of the world substantially altered sleep timing.

The diverse purposes and mechanisms of sleep are the subject of substantial ongoing research. Sleep seems to assist 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 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.

Humans may suffer from a number of sleep disorders. These include

Sleep is divided into two broad types: rapid eye movement (REM sleep) and non-rapid eye movement (NREM or non-REM sleep). Each type is associated with a distinct set of physiological and neurological features. REM sleep is associated with dreaming, desynchronized and faster brain waves, loss of muscle tone, and suspension of homeostasis. REM and non-REM sleep are so different that physiologists classify them as distinct behavioral states. In this view, REM, non-REM, and waking represent the three major modes of consciousness, neural activity, and physiological regulation. According to the Hobson & McCarley activation-synthesis hypothesis, proposed in 1975–1977, the alternation between REM and non-REM can be explained in terms of cycling, reciprocally influential neurotransmitter systems.

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