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Sleeping
41 from this deepest phase of sleep. Bed-wetting, sleep-talking and sleep-walking occur only in this stage.
Forty minutes have passed. For the next half hour or so, this sleepcycle will run backwards. From stage 4 back to stage 1. The first dream of the night is about to begin, the sleeper has entered the REM period. In REM, the vital signs change suddenly and dramatically. Breathing, heart-beat and blood pressure become irregular. Under closed lids, the eyes dance back and forth as though the sleeper were watching a movie. In theory, this is exactly what happens during REM. According to the "Scaning theory", the eyes move around as they follow the action of a dream. During REM, the brain sends a signal to the arms, legs and other large muscles to stop moving. This sleep-paralysis prevents the body from acting out movements occurring in dreams.
The first REM period lasts nearly ten minutes. When it ends, the whole cycle repeats itself, usually four or five times each night. Each cycle lasts an average of ninety minutes. As the night wears on, REM periods lengthen, while NREM periods grow shorter. The final REM period of the night may last as long as one hour, or one-half to two-thirds of the total REM sleep each night.
If a person is woken up during active sleep he will often report that he had been dreaming, whereas this is less common when someone is woken out of quiet sleep. Even a few minutes after a period of active sleep he will not remember that he had dreamt. Here then we have a means of discovering something about the relations between the electrical activity of the brain and the dreaming mind, for surely we regard dreaming as a mental activity. These two types of sleep have been found in nearly all mammals and birds, but not apparently in reptiles or other lower vertebrates or in invertebrates, though many of these show periods of inactivity. 3. Quiet sleep
The program of sleep that is written in the brain thus involves a much more complicated score than would be needed for a mere alternation of sleeping and waking.
