Your Brain: The Missing Manual

REM Sleep

REM sleep is named after the rapid eye movements that sleepers experience. Unlike other stages of sleep, REM sleep is easy to identify. When you're experiencing REM sleep, your eyes dart back and forth under your eyelids. However, the rest of your body is essentially paralyzed, which acts as a safeguard to prevent you from acting out particularly violent dreams.

REM sleep is closely identified with the phenomena of dreaming. If someone wakes you out of REM sleep, you're sure to report a vivid dream. However, other sleep states also produce dreams. Usually, these are fuzzier, more sedate dreams, and often they're little more than general feelings and soft-focus visions. But occasionally, vivid dreams are reported in non-REM sleep, most commonly at the end of a long sleep indulgence (say, a Sunday morning).

Current science suggests that our biological drive to rest just might have less to do with the tender ministrations of sleep, and more to do with the freewheeling chaos of dreams. Here are some tantalizing reasons to think REM sleep is a critical part of every brain's night:

• When deprived of REM sleep (for example, by being repeatedly woken up in the middle of a sleep cycle), the brain fights back, plunging itself into REM sleep more quickly.

• If you don't get your normal amount of REM sleep in a night, you're brain alters its sleep cycle the next night, spending more time in REM sleep to compensate.

• Adults spend about 20 percent of their sleeping hours in REM sleep. Newborns ...