Rapid eye movement (REM) 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. REM sleep normally occurs close to morning.