Rapid eye movement

R.E.M. is the abbreviation for Rapid Eye Movement. It is the part of the sleep during which people dream. It is named after a characteristic of this part of sleep, which is much different from deep sleep. During a night of sleep, a person usually has about four or five periods of REM sleep, which are quite short at the beginning of the night, but long at the end. It is believed that REM sleep, and in particular dreaming, is necessary to process the experiences we have had during the day. REM sleep is so physiologically different from the other phases of sleep that they are collectively referred to as Non-REM sleep.

The association between dreaming and REM sleep was discovered by Eugene Aserinsky and Nathaniel Kleitman in 1953.