Switching off the activity of the sympathetic nervous system enhances erections. Nocturnal erections are a good example of this. Nocturnal erections occur primarily during rapid eye movement (REM) sleep, the stage in which dreaming occurs. During REM sleep, sympathetic neurons are turned off in the locus coeruleus, a specific area of the brain stem. According to one theory, when the sympathetic nervous system is at rest, proerectile pathways predominate and allow nocturnal erections to occur. We often refer to these events as a “battery-recharging” mechanism for the penis, because they increase blood flow to the penis and thus bring oxygen to reenergize it. Studies show that women also experience episodes of nocturnal arousal when the sympathetic nervous system is a rest. Approximately four or five times a night, or during each period of REM, women experience labial, vaginal, and clitoral engorgement.