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There are 2 basic types of sleep: NREM, (non rapid eye movement) and REM (rapid eye movement) sleep.
NREM and REM sleep alternate during a normal sleep cycle. The beginning of sleep cycle is dominated by NREM sleep which gradually takes us into a deep state of unconciousness in which there is diminished brain activity, allowing blood to flow to muscles and organs. Researchers believe that during this period that the body has a chance to recover from the stress and strain of the days activities and replenish cells for the new day.
Unlike NREM sleep, REM sleep is characterized by an increase in brain activity and a strong and commensurate increase in blood flow to the brain. During REM sleep, scientists discovererd, the eyes move rapidly even though the eyelids are closed. REM sleep first appears, briefly, at around an hour and a half in to the sleep cycle, and then continues to rotate with NREM sleep appearing for greater lenghts of time as the night progresses. REM sleep periods and NREM sleep consist of more stages,which are explained afterwards. It is during the REM sleep that we dream, but researchers found out that dreaming is also possible in NREM sleep, but then much more passive. Moreover dreams launched during any of NREM sleep\'s four distinct stages tend to be shorter and more mundane than the often vivid and bizarre REM dreams.
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