During sleep the body and the brain rest and rejuvenate. Scientists can record brain activity during sleep with a machine called an electroencephalograph. The EEG shows two basic patterns of activity, rapid eye movement or REM sleep, and non-rapid eye movement, or NREM sleep. Both types of sleep are necessary for optimal health, yet have very different effects on the body. Generally, REM sleep rejuvenates the brain and NREM rejuvenates the body.
Cycles of Sleep
The body alternates between the two main sleep cycles, REM and NREM. Usually when a person first falls asleep he will go into NREM sleep, which can actually be broken down into 4 stages lasting about 90 minutes. REM sleep occurs next, and can last anywhere between 10 minutes to an hour. During the night the body will cycle between NREM sleep and REM, but spend most of the time in NREM sleep.
Brain and Body
It is believed that during REM sleep the brain is rejuvenating and resting. During NREM sleep the body is healing, regenerating tissue and building bone and muscle. When a person is not getting enough REM sleep they become confused, depressed or irritable. A person who is not getting enough NREM sleep will be tired, listless and may have a lower immune response.
During REM sleep, the eye muscles twitch, moving quickly under the eyelids. During NREM sleep the eyes are still. Scientific studies show the eyes move in different patterns depending on the REM sleep state. Sometimes the eyes roll slowly during near wakefulness. Then there are bursts of rapid eye movement and deep sleep. Scientists believe the speed of eye movement is related to scanning the action of a dream. Most of our dreams occur during REM sleep when the brain is active and body is quiet. The muscles of the body are in a state of paralysis during our dreams. If the muscles do become active during REM sleep, with sleeper may walk, talk or act out their dream.
Non-rapid eye movement sleep has four stages. The first stage of NREM occurs shortly after a person falls asleep and if woken during this beginning stage, they may not realize they had even fallen asleep at all. Hypnic myoclonia, or an intense body twitch, might occur during the first part of NREM sleep just before the muscles relax. The next three phases of NREM sleep will be deep or light, and the muscles of the body may completely relax or may contract and move between phases. During NREM sleep the brain is quiet. Generally, the sleeper will not be dreaming and her body, though relaxed, might move or switch positions between phases. The heart rate slows, body temperature decreases and growth hormones are released.