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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.[1] However, its purposes are only partly clear and are the subject of intense research.[2] 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) 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 four stages of NREM, the order normally being In humans this cycle is on average 90 to 110 minutes,[3] 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.
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