Rapid eye movement sleep (REM sleep) is a normal stage of sleep characterized by the rapid and random movement of the eyes. REM sleep is classified into two categories: tonic and phasic. It was identified and defined by Nathaniel Kleitman, Eugene Aserinsky, and Jon Birtwell in the early 1950s. Criteria for REM sleep includes rapid eye movement, but also low muscle tone and a rapid, low-voltage EEG; these features are easily discernible in a polysomnogram, the sleep study typically done for patients with suspected sleep disorders.
REM sleep in adult humans typically occupies 20–25% of total sleep, about 90–120 minutes of a night's sleep. REM sleep normally occurs close to morning. During a normal night of sleep, humans usually experience about four or five periods of REM sleep; they are quite short at the beginning of the night and longer toward the end. Many animals and some people tend to wake, or experience a period of very light sleep, for a short time immediately after a bout of REM. The relative amount of REM sleep varies considerably with age. A newborn baby spends more than 80% of total sleep time in REM. During REM, the activity of the brain's neurons is quite similar to that during waking hours; for this reason, the REM-sleep stage may be called paradoxical sleep.
REM sleep is physiologically different from the other phases of sleep, which are collectively referred to as non-REM sleep (NREM sleep). Subjects' vividly recalled dreams mostly occur during REM sleep.
Read more about Rapid Eye Movement Sleep: Physiology, REM Sleep in Animals, Theories About The Function of Rapid Eye Movements, Discovery
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