Every guy has three to six erections a night, each lasting anywhere from 10 to 40 minutes.
These involuntary nocturnal erections appear during REM (Rapid Eye Movement) sleep, the stage in which dreaming occurs. But they have nothing to do with the dream content. A nocturnal hard on doesn’t imply that you are sexually excited.
Nightly erections are like a ‘battery-recharging’ mechanism for the penis. The pelvic blood flow ensures the penis gets all the oxygen it needs.
Women also experience episodes of nocturnal arousal during REM sleep. Approximately four or five times a night they experience labial, vaginal and clitoral engorgement.
Normally guys don’t notice these nocturnal erections. Only when a wet dream wakes them up or they wake up in the morning with a hard on.