Whether he's dreaming about sex, red pickup trucks, fresh artichokes or going to class in his underwear, every healthy male experiences spontaneous, involuntary erections as he dreams. They occur every 90 minutes or so during the dream stage of sleep. Scientists aren't certain why sleep erections occur, but one prevailing theory is that they are a natural systems check or some kind of exercise program to keep the penis in shape. A woman might interpret morning wood as an invitation for sex. That's a good instinct. But the guy's immediate thought is probably, How am I going to pee with this thing? Women apparently experience similar sleep arousal. But while blood flow to the penis can be monitored by attaching an expandable ring, measuring blood flow to a woman's genitals is more of a challenge. Sleep erections are typically monitored to determine if erectile dysfunction is a physical or a mental problem.