Rapid eye movement sleep is a stage of sleep characterized by the rapid and random movement of the eyes. Rapid eye movement sleep is classified into two categories: tonic and phasic. It was identified and defined by Nathaniel Kleitman and his student Eugene Aserinsky in 1953. Criteria for REM sleep includes rapid eye movement, 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 typically occupies 20–25% of total sleep, about 90–120 minutes of a night’s sleep. The first REM sleep period occurs 90-120 min after sleep onset with the last REM period usually being the longest and normally occurs close to morning. During a night of sleep, one usually experiences 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.