Rapid eye movement (REM) sleep is the normal stage of sleep characterized by rapid movements of the eyes. REM sleep is classified into two categories: tonic and phasic. It was discovered by Nathaniel Kleitman and Eugene Aserinsky in the early 1950s. Their seminal article was published September 4, 1953 (Aserinsky E, Kleitman N. Regularly Occurring Periods of Eye Motility, and Concomitant Phenomena, during Sleep. Science 1953:118;273-274). Criteria for REM sleep include not only rapid eye movements, 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 adults typically occupies 20-25% of total sleep, lasting about 90-120 minutes. During a normal night of sleep, we usually experience about 4 or 5 periods of REM sleep; they are quite short at the beginning of the night and longer at the end. It is common to wake for a short time at the end of a REM phase. The relative amount of REM sleep varies considerably with age. A newborn baby spends more than 80% of total sleep time in REM (see also Active Sleep). During REM, the summed activity of the brain's neurons is quite similar to that during waking hours; for this reason, the phenomenon is often called paradoxical sleep. This means that there are no dominating brain waves during REM sleep.
REM sleep is so physiologically different citation needed] from the other phases of sleep that the others are collectively referred to as non-REM sleep. Most of our vividly recalled dreams occur during REM sleep.