It’s 4 a.m. and you wake up with an erection. What gives?

This phenomenon, also known as morning wood, is so common that doctors actually have a medical name for it: nocturnal penile tumescence (NPT).

While you might notice this more frequently in the wee hours, these erections can occur anywhere from three to five times throughout the night.

“It’s typical of adolescents and young men, due to their peak levels of hormones, which correlate to the frequency of their erections and their need for release,” says Gloria Brame, Ph.D., sexologist and author of Sex for Grown-Ups. “Naturally, as men age, this occurrence wanes as their testosterone levels and sexual function decline.”

So what’s causing them in the first place?

There are plenty of old wives’ tales suggesting your morning erections are caused by sexy dreams, but experts aren’t convinced.

“Morning wood has nothing to do with dreams during the night, as spontaneous erections have also been observed in male fetuses in the womb,” says Uta Demontis, Ph.D., clinical sexologist and relationship coach.

With those ruled out, Demontis says that it’s possible that the male body spontaneously produces erections in the middle of the night to oxygenate his penis with some fresh blood flow.

Other experts believe that because all humans are really out of it during REM sleep, the penis is finally free to have a life of its own and chooses to be hard.

“I mainly attribute morning wood to hormonal spikes and assume that it’s those spikes that bring on the dreams or cause men to suddenly be aware that their penis is sensitive during sleep,” says Brame.

So why don’t you pop sudden erections in the middle of the day?

“The male brain emits an anti-erection chemical during their waking hours that helps keep them from springing an erection at socially inappropriate times,” says Brame.

During sleep, however, this inhibiting factor goes away, allowing erections to come and go freely, she says.

The article Why Are Men So Horny in the Morning? originally ran on WomensHealthMag.com.