Sleep is the natural state of bodily rest observed in humans and other animals. It is distinguished from quiet wakefulness by a decreased ability to react to stimuli while it is more easily reversible than coma. It is common to all mammals and birds, and is also seen in many reptiles, amphibians, and fish. In humans, other mammals, and a substantial majority of other animals that have been studied (such as some species of fish, birds, ants, and fruit flies), regular sleep is essential for survival. The purposes of sleep are only partly clear and are the subject of intense research.

Stages of sleep

In mammals and birds, sleep is divided into two broad types: Rapid Eye Movement (REM) and Non-Rapid Eye Movement (NREM or non-REM) sleep. Each type has a distinct set of associated physiological, neurological, and psychological features. The American Academy of Sleep Medicine (AASM) further divides NREM into three stages: N1, N2, and N3, the last of which is also called delta, or slow-wave, sleep (SWS). Sleep cycles through the night, with deep sleep early on and more REM (marked in red) toward morning. thumb|165px|right|Stage N3 sleep. EEG highlighted by red box.

Sleep proceeds in cycles of REM and NREM, the order normally being N1 → N2 → N3 → N2 → REM. There is a greater amount of deep sleep (stage N3) early in the night, while the proportion of REM sleep increases later in the night and just before natural awakening.

The stages of sleep were first described in 1937 by Alfred Lee Loomis and coworkers, who separated the different EEG features of sleep into five levels (A to E), which represented the spectrum of wakefulness to deep sleep. In 1953, REM sleep was discovered as distinct, and thus William Dement and Nathaniel Kleitman reclassified sleep into four NREM stages and REM. The staging criteria were standardized in 1968 by Allan Rechtschaffen and Anthony Kales in the "R&K sleep scoring manual." In the R&K standard, NREM sleep was divided into four stages, with slow-wave sleep comprising stages 3 and 4. In stage 3, delta waves made up less than 50% of the total wave patterns, while they made up more than 50% in stage 4. Furthermore, REM sleep was sometimes referred to as stage 5.

In 2004, the AASM commissioned the AASM Visual Scoring Task Force to review the R&K scoring system, which culminated in several changes, the most significant being the combination of stages 3 and 4 into Stage N3. This was published in 2007 as The AASM Manual for the Scoring of Sleep and Associated Events. Arousals and respiratory, cardiac, and movement events were also added.

Sleep stages and other characteristics of sleep are commonly assessed by polysomnography in a specialized sleep laboratory. Measurements taken include electroencephalography (EEG) of brain waves, electrooculography (EOG) of eye movements, and electromyography (EMG) of skeletal muscle activity. In humans, each sleep cycle lasts from 90 to 110 minutes on average, and each stage may have a distinct physiological function. Drugs such as sleeping pills and alcoholic beverages can suppress certain stages of sleep, leading to sleep deprivationFact: date=January 2009. This can result in sleep that exhibits loss of consciousness but does not fulfill its physiological functions (i.e., one may still feel tired after otherwise sufficient sleep). REM and slow-wave sleep are both homeostatically driven; people and most animals selectively deprived of one of these stages will rebound once uninhibited sleep is allowed.Fact: date=March 2009 This finding suggests that both these stages are essential.