Rapid eye movement sleep or REM sleep is a phase of sleep where
- The closed eyes move rapidly and randomly beneath the closed eyelid in a manner to be clearly visible to an observer;
- The sleeping person lacks muscle tone, such as the ability to remain seated in an upright position; and
- An EEG exhibits a rapid low-voltage pattern.
A polysomnigraph is an instrument that is designed to easily detect these signs without a human observer.
About 20-25% of overall adult sleep is REM sleep. Typically, a person has about four or five episodes of REM sleep in a typical night's sleep, with the length of REM sleep increasing the longer a person has been asleep (or is closer to waking). The amount of REM sleep decreases as a person ages with infants spending about 80% of their sleep time in REM sleep.
During REM sleep, the brain is much more active than it is during other periods of sleep, being almost as active as when a person is fully conscious. The body's physiology is also quite different than during the rest of sleep - certain neurotransmitters stop working and the body is nearly entirely paralyzed. In addition, it appears that most of the dreams a person remembers happens during this period of sleep.
The purpose of REM sleep is not well understood. However, it is clear that a person who does not have REM sleep will start to suffer symptoms typical of complete sleep deprivation, such as depression and poor concentration.