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Sleep stages

http://web.mst.edu/~psyworld/general/sleepstages/sleepstages.pdf

http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=2335403

COPIED FROM SLEEP, 04 JANUARY 2009:

Physiology
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.

Sleep proceeds in cycles of REM and the three stages of NREM, the order normally being:
 * stages N1 -> N2 -> N3 -> N2 -> REM.

(Until recently, NREM was considered to include four stages and the cycle was N1 -> N2 -> N3 -> N4 -> N3 -> N2 -> REM; this system is still seen in reference works and diagrams.)

In humans each sleep cycle lasts on average 90 to 110 minutes, with a greater amount of stage 3 (N3) 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, leading to sleep deprivation. This can result in sleep that exhibits loss of consciousness but does not fulfill its physiological functions.



Stages
Criteria for staging or scoring the stages of sleep, have been changed several times. First described in 1937 by Loomis et al, the staging was changed in 1957 after the discovery of rapid eye movement (REM) sleep, standardized in 1968 by Rechtschaffen & Kales (R&K) and most recently in 2007 by The AASM Manual for the Scoring of Sleep and Associated Events published by the American Academy of Sleep Medicine (AASM). In addition to adding arousals and respiratory, cardiac, and movement events, the major change in 2007 was the elimination of Stage 4: in the R&K standard, the difference between Stages 3 and 4 (now combined as 3) was that delta waves made up less than 50% of the total wave-patterns in stage 3, while they were more than 50% in stage 4. The R&K standard included four stages of NREM sleep plus REM, sometimes referred to as "stage 5". The newer standard discontinues stage 4 sleep and leaves only stage 3 to describe deep sleep, also known as delta or slow-wave sleep.

Sleep stages and other characteristics of sleep can be measured by polysomnography in a sleep laboratory using among other tools electroencephalography (EEG) for brain waves, electrooculography (EOG) for eye movements and electromyography (EMG) for activity of skeletal muscles.

Criteria for REM sleep include not only rapid eye movements but also rapid low voltage EEG, commonly called brain waves. In mammals, at least, low muscle tone is also seen, often called paralysis. Most memorable dreaming occurs in this stage. REM sleep accounts for 20–25% of total sleep time in normal human adults; NREM (non-REM) accounting for the rest. In NREM sleep, there is relatively little dreaming.

Non-REM encompasses three stages; stage 1 (N1), stage 2 (N2), and stage 3 (N3); N3 being referred to as 'deep sleep' or slow-wave sleep (SWS). Sleep stages are differentiated by a combination of brain waves, eyes movements, and skeletal muscle activity. In non-REM sleep there may be limb movements, and parasomnias such as sleepwalking may occur.

NREM consists of three 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 (with a 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 troublesome to them. During N1 the subject loses some muscle tone and most 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 EMG decreases and conscious awareness of the external environment disappears. This stage occupies 45 to 55% of total sleep in adults.
 * Stage N3, deep or slow-wave sleep (SWS), is characterized by delta waves, (0.5 to 4 Hz), also called delta rhythms. This is the stage in which night terrors, bedwetting, sleepwalking and sleep-talking occur.

REM sleep and SWS sleep (previously stages 3 and 4, now combined as stage 3) are both homeostatically driven; that is, people and most animals selectively deprived of one of these stages will rebound once uninhibited sleep is allowed. This finding suggests that both types of sleep are essential.