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In the 1950s American
physiologists Eugene Aserinsky and Nathaniel Kleitman reported that
periods of eye movement and twitching occur during sleep. They named
these periods rapid eye movement (REM) sleep. Aserinsky and Kleitman
found that when subjects were awakened during REM sleep, they
reported vivid dreams. Scientists believe that REM sleep is closely
related to wakefulness because brain wave activity during REM sleep
is marked by short, rapid wave patterns similar to brain wave
activity of the waking state.
Sleep characterized by little or no eye movement is called nonrapid
eye movement (NREM) sleep. During NREM sleep, breathing and heart
rates slow down, and body temperature and blood pressure often
decrease. When awakened from periods of NREM sleep, subjects are
much less likely to report vivid, action-packed dreams. Brain wave
activity during NREM sleep is dominated by large, slow waves that
contrast markedly to the short, rapid wave patterns characteristic
of REM sleep and the waking state.
Sleep studies based on EEGs have shown that during a normal night,
humans cycle between REM sleep and NREM sleep in very regular
patterns. In adults aged 20 to 60, REM sleep occurs about every 90
minutes. In this 90-minute cycle, humans fall into progressively
deeper stages of NREM sleep, then cycle back through the stages
until they enter REM sleep, and then the cycle begins again. In a
normal night, the number of REM periods varies from four to six,
depending on the length of the episodes and the total time asleep.
REM episodes in the beginning of the night usually last about ten
minutes and, during the night, grow progressively longer, lasting up
to 30 minutes in the early hours of the morning. Most adults spend
about 20 percent of their total sleep time in REM sleep. |
