Other interesting physiological factors appear during REM sleep. Measurements of other parts of the body during sleep show that everything is fairly slow and even during NREM sleep, but as soon as REM appears, things change in the physical body. The respiration speeds up, spinal pressure changes, muscle tone changes, and genital arousal happens, interestingly enough. Genital arousal, for men and women, is present in 95 % of the REM periods. At first, only men were studied in the laboratory, rather than women, for erectile function was easier to observe. Studies were done on women later as the results came in for the men.

Sixty percent of the male's REM time was spent with a full erection and 35% was spent with partial erections. Quite amazingly, children between the ages of three and nine had the highest percentage of full erection, this decreased as age progressed until in the elderly it was discovered to be half that amount of time. These erectile periods seemed to come right before REM starts and linger after REM ends.

It was discovered that subjects who had impotence due to a physical reason like nerve damage or diabetes did not have erections during REM periods. However, those who were impotent due to psychological reasons did have normal full erections during sleep, even though they could not obtain an erection during waking hours.

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