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Two kinds of sleep TWO KINDS OF SLEEP
With the help of the sleep laboratory we can now chart and record sleep patterns and study them later with the help of a computer. Two kinds of sleep are identified. The first kind is REM sleep, during which the eyes move rapidly under the eyelids and dreams are experienced. In the second kind of sleep there is no rapid eye movement. This is called non-REM sleep or NREM sleep. This NREM sleep consists of the four stages of sleep.
Sleep cycles. When a person falls asleep, he goes into NREM sleep, first entering the short-lived stage 1 sleep and then passing into stage 2 sleep. Stage 2 is the main stage and occupies about 50 per cent of the time spent in NREM sleep. He then goes into stages 3 and 4 of slow wave sleep. After 90 minutes from the onset of sleep he goes back to stage 2 sleep and enters the first REM sleep. The time up to the beginning of the first REM sleep is called the first sleep cycle.
The second sleep cycle starts with the first REM sleep, continues through the four stages of NREM sleep, and ends at the beginning of the next REM sleep. Hence each sleep cycle consists of some REM sleep and some NREM sleep, except for the first sleep cycle which has little or no REM sleep.
Each sleep cycle lasts about 90 minutes and, as the night progresses, each REM period, and consequently each dream experience, lasts longer and longer, with the last REM stage in the morning perhaps lasting over an hour. This is why, when we wake up in the morning, we are very often in the middle of a dream. A normal sleep pattern will have four or five sleep cycles a night.
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