Waking up first thing in the morning with an erection is a pretty common experience, but the cause behind it isn’t as widely known. In a new video on her YouTube channel, urologist, surgeon and sexual health educator Dr. Rena Malik breaks down the phenomenon of “nocturnal penile tumescence”—or as everyone else calls it, morning wood.
Evidence of involuntary erections goes all the way back to Ancient Greece, with the philosopher Plato writing: “In males the nature of the genital organs is disobedient and self-willed, like a creature that is deaf to reason, and it attempts to dominate all because of its frenzied lusts.”
In the modern day, research has found that erections occur throughout the four or five cycles of REM sleep that everybody goes through at night—with both men and women experiencing either penile or clitoral erections. That boner you have when you wake up, Malik explains, is simply the last one in this sequence of sleep-related erections.
These erections happen because of four key factors. Firstly there’s the parasympathetic nervous system, which is responsible for rest and digestion. During sleep, this system causes the body’s muscles to relax, slowing the heart rate and increasing blood flow to the penis. Secondly there are the hormonal changes that take place while you’re asleep, with testosterone levels rising as you get closer to waking. The third reason is simple friction against the penis due to sleeping position, sheets, or night clothes. And finally, there’s the fact that your bladder is usually full when you wake up: this can lead to compression of the sacral nerves, which can cause a reflexive erection.
What’s most important to remember, though, is that getting an erection in the morning is nothing to worry about. In fact, if anything, it’s a good thing.
“These night-time erections are actually a sign of good blood flow to the penis, and a sign of good erectile health,” says Malik, adding that research shows they may even support ongoing erectile function: “In the erect state, you increase oxygenation to those tissues, and so having sleep-related erections is actually good.”
This article was first published on Men’s Health US.