Definition: Normally, a healthy person experiences two sleep cycles or primary stages of sleep at night. These include REM (rapid eye-movement sleep) and Non-REM sleep. Usually when someone starts to fall asleep they are in the first stage or non-REM sleep. There are three phases of non-REM sleep including light sleep, regular sleep and deep sleep sometimes called "delta" sleep). As a person falls into delta or heavy sleep, gradually their body shifts into the next sleep stage, REM or rapid eye-movement sleep. During this stage of sleep, the brain is active and people dream.
Researchers have shown the brain is as active during REM sleep as it is when awake, however most people are not able to move large muscles in the body during this stage of sleep. In fact, during REM sleep, most people experience a temporary paralysis of major muscle groups, so they can't move. If you were to watch someone during REM sleep however, you might see their eyelids shifting frequently, as they dream. While researchers do not understand REM sleep completely, they do know it is important because it may help refresh the body and prevent psychological and behavioral problems associated with sleep deprivation or getting too little sleep.
Also Known As: sleep phases, stages of sleep, REM, Non-REM sleep, healthy sleep
Examples: During a given night, a person will experience two distinct phases of sleep, and multiple stages of sleep within each phase.