Sleep is a natural state of bodily rest observed throughout the animal kingdom. 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 which have been studied — such as fish, birds, ants, and fruit-flies — regular sleep is essential for survival. However, its purposes are only partly clear and are the subject of intense research.

Physiology

In mammals and birds the measurement of eye movement during sleep is used to divide sleep into the two broad types of Rapid Eye Movement (REM) and Non-Rapid Eye Movement (NREM) sleep. Each type has a distinct set of associated physiological, neurological and psychological features.

Sleep proceeds in cycles of REM and the four stages of NREM, the order normally being:

stages 1 -> 2 -> 3 -> 4 -> 3 -> 2 -> REM.

In humans this cycle is on average 90 to 110 minutes, with a greater amount of stages 3 and 4 early in the night and more REM later in the night. Each phase may have a distinct physiological function. Drugs such as sleeping pills and alcoholic beverages can suppress certain stages of sleep (see Sleep deprivation). This can result in a sleep that exhibits loss of consciousness but does not fulfill its physiological functions.

Allan Rechtschaffen and Anthony Kales originally outlined the criteria for identifying the stages of sleep in 1968. The American Academy of Sleep Medicine (AASM) updated the staging rules in 2007.

Stages of sleep

Criteria for REM sleep include not only rapid eye movements but also a rapid low voltage EEG. In mammals, at least, low muscle tone is also seen. Most memorable dreaming occurs in this stage. NREM accounts for 75–80% of total sleep time in normal human adults. In NREM sleep, there is relatively little dreaming. Non-REM encompasses four stages; stages 1 and 2 are considered 'light sleep', and 3 and 4 'deep sleep' or slow-wave sleep, SWS. They are differentiated solely using EEG, unlike REM sleep which is characterized by rapid eye movements and relative absence of muscle tone. In non-REM sleep there are often limb movements, and parasomnias such as sleepwalking may occur. A cyclical alternating pattern may sometimes be observed during a stage.

NREM consists of four stages according to the 2007 AASM standards:

• During Stage N1 the brain transitions from alpha waves (having a frequency of 8 to 13 Hz, common to people who are awake) to theta waves (frequency of 4 to 7 Hz). This stage is sometimes referred to as somnolence, or "drowsy sleep". Associated with the onset of sleep during N1 may be sudden twitches and hypnic jerks also known as positive myoclonus. Some people may also experience hypnagogic hallucinations during this stage, which can be more troublesome to the subject. During N1 the subject loses some muscle tone and conscious awareness of the external environment.
• Stage N2, is characterized by "sleep spindles" (12 to 16 Hz) and "K-complexes." During this stage, muscular activity as measured by electromyography (EMG) lowers and conscious awareness of the external environment disappears. This stage occupies 45 to 55% of total sleep.
• In Stage N3, the delta waves (0.5 to 4 Hz), also called delta rhythms, make up less than 50% of the total wave-patterns. This is considered part of deep or slow-wave sleep (SWS) and appears to function primarily as a transition into stage N4. This is the stage in which night terrors, bedwetting, sleepwalking and sleep-talking occur.
• In Stage N4, delta-waves make up more than 50% of the wave-patterns. Stages N3 and N4 are the deepest forms of sleep; N4 is effectively a deeper version of N3, in which the deep-sleep characteristics, such as delta-waves, are more pronounced. As of new AASM guidelines, the determination between stage 3 and stage 4 sleep is inconsequential and both may be considered delta sleep or slow wave sleep. Therefore, in a recent ruling by the AASM, in order to make precision the scoring guidelines, stage four had been disbanded, and left is the stage of sleep N3 to describe the delta sleep attributed to it.