of REM sleep is a protective reflex. During this sleep, we’re not conscious of the outside world, and current theories suggest that our brains are rehashing all the information received during the day, as well as analyzing memories and forming brand new ones. During this work, frightening or dangerous thoughts can occur, and the paralysis of our muscles prevents us from acting on those thoughts and dreams.

People with a condition called REM Sleep Behavioral Disorder, which interferes with REM paralysis, can respond to a dream with dangerous movement. For example, if they’re having a dream in which someone is chasing them, they can jump out of bed and start running away—and crash into a bedside table or fall. If they’re dreaming that they’re in a fight with someone, they can punch or kick the person sleeping next to them. If we were able to move during REM sleep and act on our dreams, we could end up in the hospital—or always have to sleep alone.


Because REM sleep combines a very active brain with a paralyzed body, it is sometimes called paradoxical sleep.

REM sleep appears to be particularly important to the developing child and adolescent. Studies on the effects of sleep deprivation suggest that REM sleep deprivation in newborns can negatively affect central nervous system development, and permanent sleep disruption early in life can cause an abnormal number of neural cells to die. REM sleep is also crucial for the anabolic, or energy building and healing, activities that take place in the brain and in the body.

The Cycles of Sleep

Once you’ve snoozed your way from boring-lecture sleep through REM sleep, you’re said to have completed one cycle of sleep. A full cycle takes approximately 90 to 120 minutes, so if you get all the rest you need, you’ll go through four or five cycles each night.

But on the second through the fourth or fifth cycles, things change a bit. After you complete REM sleep, you may only briefly pass through boring-lecture sleep or skip it entirely before moving into Stage 2 and then completing the stages through Stage 5 again. The time you spend in each stage also changes; the first half of the night is spent mostly in

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