Wednesday, March 04, 2009

Sleep

Sleep is the natural state of bodily rest observed in humans and other animals. 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 some species of fish, birds, ants and fruit-flies, regular sleep is essential for survival.

A widely publicized 2003 study performed at the University of Pennsylvania School of Medicine demonstrated that cognitive performance in humans declines with fewer than eight hours of sleep. However, the purposes of sleep are only partly clear and are the subject of intense research.

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 in 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.

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