Many men think their penises are erect upon waking because of urine buildup in the bladder, but this is false. Morning erections are technically nighttime erections which happen during Rapid Eye Movement (REM) sleep (when most dreaming occurs), although men older than sixty years may have them during even non-REM sleep. Generally, we experience four intervals of REM sleep per night. These spontaneous woodies in the wee hours are caused by specific neuroreflexes that are stimulated during REM sleep.

Most guys have nocturnal erections throughout their lifetime, and their frequency, along with one's awareness of them, may vary. Nighttime/morning erections can be noticeably harder or stiffer than those had when awake. The penis can stay erect for up to two hours nightly, and not all men who have erections during the night wake up with one in the morning.