Sex Ed

Why Men Get Erections in Their Sleep and Often Wake Up with ‘Morning Wood’

March 8, 2021 by Justin Lehmiller

As a sex educator, I’ve been asked many times over the years why men often wake up with erections (or “morning wood,” as they’re known colloquially). In this post, I’ll explore why, and also what happens to the penis more generally during sleep.

People who were born with penises—regardless of whether they identify as men—experience an average of 4-5 erections during a full night of sleep. Little-known fact: people born with clitorises—again, regardless of their gender identity—experience the same thing.

These sleep erections last, on average, between 25-35 minutes each. This is something that happens throughout life, beginning in infancy and carrying through adulthood.

These erections occur specifically during rapid eye moment (REM) sleep. Humans undergo 4-5 periods of REM sleep each night (hence why there are 4-5 erections), and we spend about one-quarter to one-fifth of a total night of sleep in the REM state. If you happen to wake up during one of these times, well, that’s where morning wood comes from.

There’s a lot we still don’t understand about REM sleep, but we do know that this is when dreams tend to occur. Our brains happen to be more active during this time (in fact, they reach a level of activity that is similar to being awake). However, it is not the content of the dreams themselves that causes nighttime erections. Although sex dreams and nocturnal erections can potentially co-occur, one does not necessarily cause the other.

Instead, it is thought that nocturnal erections are due to hormonal and neurotransmitter effects that occur during REM sleep. For example, there are changes in testosterone and prolactin levels during sleep, and they peak during the REM cycle. Both of them are known to play a role in sexual and reproductive functions.

At the same time, there is also enhanced release of acetylcholine and lower release of norepinephrine in certain brain regions—and both of these neurotransmitters are known to play a role in regulating erections.

The overall pattern is such that there tend to be more stimulators of erection present in the brain during this time.

The erections that occur during REM sleep are considered to be normal and healthy. A lack of sleep-related erections is actually one of the most common indicators of severe erectile dysfunction—and this is a key way that doctors can distinguish between ED that has physical versus psychological causes. In other words, if everything is functioning normally during sleep, that’s a sign that erection problems during waking periods probably have a psychological cause.

So what’s the purpose of these nighttime erections? There are a few schools of thought. One is that this is a way of promoting genital health. Specifically, these erections may help to keep tissue functional and healthy by promoting more oxygenation.

However, another hypothesis that has been suggested is that there may be a potential sexual and reproductive function: “men’s REM sleep-related erections during sleep might have evolved to allow procreation irrespective of their will or intention” (Youn, 2017). In other words, given that we spend so much of our lives sleeping, maybe the human body evolved to potentially enable procreation during this time? Whether that’s actually true, we can’t say with any certainty—it’s just another idea that has been proposed and, not surprisingly, it is quite controversial.

For a more detailed review of the research on nocturnal erections, see: Youn, G. (2017). Why Do Healthy Men Experience Morning Erections? The Open Psychology Journal10(1).

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Image Source: 123RF/Roman Stetsyk

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Written by
Dr. Justin Lehmiller
Founder & Owner of Sex and Psychology

Dr. Justin Lehmiller is a social psychologist and Research Fellow at The Kinsey Institute. He runs the Sex and Psychology blog and podcast and is author of the popular book Tell Me What You Want. Dr. Lehmiller is an award-winning educator, and a prolific researcher who has published more than 50 academic works.

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