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Sleep is a bodily state in which the body and mind rest. Almost all animals must sleep regularly to survive. However, sleep seems to play in important role in growth and memory processing, and sleep deprivation can lead to immune system deficiencies and a significant decrease in judgment and reaction time.

Sleep occurs in several stages, which vary in the type and intensity of the brain activity that occurs, which is measured via EEG. Stage 1 sleep is characterized primarily by an alpha and theta EEG waves. During stage 2 theta waves predominate, and the sleeper becomes less aware of the environment. Stages 3 and 4 are also referred to as slow-wave sleep, and are characterized by a delta wave EEG pattern. Stages 1 - 4 are also referred to as NREM (non rapid eye movement) sleep, and account for the majority of sleep time. Following stage 4, a person enters REM (rapid eye movement) sleep, which as the name suggests is characterized by rapid movements of the eyes and increased brain activity relative to the other sleep stages. Although dreams occur in other sleep stages, dreams that are remembered after waking typically occur during REM sleep. Most people cycle through the stages of sleep several times each night, with each cycle lasting about 90-120 minutes.


Since sleep is a fairly basic, but abstract, concept, the morphology of the corresponding word is useful in historical linguistics for determining fine relationships between related languages. The noun sleep and its derived verb and adjective forms to sleep, sleepy in English both originate from a common Germanic root *SLAEP, as exemplified by the many cognates of the word in other Germanic languages such as the Dutch slap, slapen, slaeperig and the German Schlaf, schlafen.