Sleep is the natural state of rest observed throughout the animal kingdom, in all mammals and birds, and in many reptiles, amphibians, and fish. In humans, other mammals, and a substantial majority of other animals which have been studied by humans ? such as fish, birds, ants, and fruit-flies ? regular sleep is necessary for survival. While sleep is essential for survival, there currently does not exist credible scientific evidence behind the purpose of sleep. Physiology (Stages of Sleep) In mammals, the measurement of eye movement during sleep is used to divide sleep into two broad types: · 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 NREM phases. In humans, this cycle is approximately 90 to 120 minutes.

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