Rapid eye movement (REM) is a physiological phenomenon which occurs during stage five sleep, the part of the sleep cycle when people dream. Some people refer to this stage of sleep as REM sleep, in a reference to the physiological markers which can be used to identify it. The earliest research on rapid eye movement was done in the 1950s in Chicago by Nathaniel Kleitman and Eugene Aserinsky, and the two revealed a number of interesting things about the sleep and cognition with their research.
Humans are not the only animals to demonstrate rapid eye movement. Birds and other mammals appear to have REM cycles in their sleep as well. The younger an animal is, the more time it spends in REM sleep; babies, for example, can spend up to seven hours a day in REM sleep. The exact function of this stage of sleep is not fully understood, although scientists have a couple of hypotheses.
As you might imagine, rapid eye movement is characterized by jerky movements of the eyes under the lids. This movement is totally involuntary, and it is typically accompanied with a still body, as the brain appears to block the neurotransmitters which allow muscles to move during REM sleep. While in REM sleep, people often have vivid and distinctive dreams which they remember when they wake up; researchers who have interrupted REM sleep often get subjects to recall up to 80% of their dreams, and these researchers have also noted that the subjects make up for the interruption with a long phase of rapid eye movement sleep.
There are five distinct stage in sleep, which are repeated over and over again. A complete cycle takes around an hour and a half, which has led some people to suggest that sleep should occur in increments of an hour and a half; in other words, six hours of sleep may actually be better than seven. The first four stages of sleep start light and get progressively deeper, with brain waves slowing dramatically. In the fifth stage, REM sleep, the brain gets extremely active, almost like the sleeper is awake.
The rapid eye movements associated with this stage can be used to identify it, but researchers can also identify REM sleep patterns by using machines which measure brain activity. The signature of REM sleep is quite distinctive, and recognizable to many sleep researchers. It is believed that this phase of sleep may allow the brain to consolidate and process memories, and it may also be a resting period for the mind, allowing it to recharge after a busy day.