摘要：People begin the sleep cycle with a period of non-rapid eye movement (NREM) sleep, followed by a very short period of rapid eye movement (REM) sleep. During a normal night of sleep, humans usually experience about four or five periods of REM sleep. Penile erections have generally been considered to be an epiphenomenon of REM sleep-related physiological changes for healthy males. Thus, men are very likely to awaken in the morning with a REM sleep-related erection, which is also known as nocturnal penile tumescence (NPT). Men who are physically under great strain or serious psychological stress may find it difficult to maintain a psychogenic erection. The best time for them to have sex would be during sleep time, such as when they are experiencing REM sleep-related erections. It is reasonable to assume that the NPT phenomena might have evolved as a tool for having sex, in the context of both procreation and recreation.
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