The Science of Morning Wood

Here's why men so often wake up with an erection (and why the same thing happens to women, too).

The Science of Morning Wood

For most men, waking up with an erection is as predictable as needing to pee in the morning. (Admittedly, when the two are combined, it does raise a sloppy conundrum.) But what is nocturnal penile tumescence (NPT) — affectionately known as morning wood — all about? Why does sleep seemingly inspire an erection in the same way as the sex act that turns you on the most? And why is it so persistent — always there, day in, day out?

Turns out there’s quite a bit of science at play here…

  1. Like vivid dreams, NPT is related to rapid eye movement sleep; it’s triggered by a different part of the brain than arousal erections. As you enter REM sleep, your brain starts to shut off some neurotransmitters in an effort to regulate your body and prevent you from acting out your dreams. One of these transmitters, norepinephrine, is involved in the control of erections. Specifically, it causes vasoconstriction of penis blood cells, actively preventing an erection — or more simply put, norepinephrine is the reason you don’t walk around with a boner all day. However, as you enter REM sleep, norepinephrine decreases and testosterone-related activities commence. This leads to vasodilation, or increased blood flow to the blood vessels, ultimately causing an erection, which is intensified by the fact that the REM period becomes prolonged as the morning approaches.
  2. Morning wood starts to occur as early as in utero. “The physiological mechanism is the same within the placenta or outside of it — the blood flow and pressure mechanisms remain the same,” urologist Dr. Muhammad Mirza explains. They can last until you die. At the very least, they remain prominent until your 40s — especially when you’re a 40-year-old virgin.
  3. Women get it, too, in the form of clitoral erections. In addition to the obvious size differential, clitoral erections are more often accompanied by an orgasm. “Scientific data suggests that the frequency of orgasm with nocturnal erections is higher in females,” Mirza says. Both nocturnal penile and clitoral erections, though, do happen at the same time (during REM sleep) and for the same reason (the absence of norepinephrine).
  4. It’s also been suggested that morning wood — whether of the male or female variety — could be the body’s way of providing adequate oxygen flow throughout the body, including the genitals.
  5. One study found most of the men surveyed weren’t aroused when they woke up with morning wood and felt their penis was ‘“in the way’” more than anything else. Point being, there’s not much sexual about morning wood. One of the 21-year-old males in the study reported: “Oddly enough, my morning erections are WAY harder and more persistent than my arousal erections. I have no idea why, but it basically always results in me giving in and beating the devil out of it or having sex if my partner is so gracious and willing in the morning.”
  6. Twenty-five-year-old Kyle agreed. “It happens at the exact same time you have to take a leak. Your choices are either uncomfortably force it into position in order to get it all in the bowl, or leave it as is and lean in and arch it — kind of like a Bellagio fountain of piss.”
  7. There’s evidence that a full bladder can contribute to morning wood. The increased bladder size through the night stimulates a region of the spinal cord, which can cause a “reflex erection.” The physiological benefit is to prevent you from wetting the bed. Some men may not always be roused instantly when nature calls, but will feel their erect penis as they awaken and realize they need to use the bathroom.
  8. Sometimes, though, the situation is far more uncomfortable (and serious) than having to awkwardly force out some urine. There’s a rare condition called intermittent nocturnal priapism in which the erections you achieve in dreamland — but not necessarily the ones you get while awake — hurt enough to shake you from your sleep. An anonymous Metafilter poster notes, “Over the past few months I’ve been waking up several times during the night with an extremely turgid painful erection that won’t go away unless I stand up and walk around for a bit, or raise my legs or squeeze my thigh muscles. When this happens, I don’t feel sexually aroused and my penis doesn’t feel responsive in any way — it seems to be purely a blood-pressure thing of some sort. What’s going on? It feels like my penis is going to explode in a bloody mess sometimes. Has anyone else had this problem? If it matters, my normal resting blood pressure is quite low — on average 110/60.”
  9. The “postage stamp test” is used to check for nocturnal erections in diagnosing male impotence. A length of connected postage stamps are secured loosely around the male’s flaccid penis just prior to sleeping. If the perforated connections between the individual stamps are torn up by the morning, this is taken as evidence of nocturnal tumescence.
  10. “Consistently having erections while you sleep indicates healthy blood flow to your penis, which also is necessary for getting hard when you’re turned on,” explains Tobias Köhler MD, an associate professor and residency program director of the urology division at Southern Illinois University’s School of Medicine. “A healthy man should expect to get hard three to five times per night. So if you experience erection problems when you’re trying to get busy — but you get them overnight or when you wake up in the morning — that points more to a psychological cause of erectile dysfunction, like performance anxiety or depression. But if you haven’t been experiencing erections at night either, that signals that blood flow to your penis may be an issue. This is a common characteristic of underlying conditions like heart disease, blocked blood vessels, high cholesterol or high blood pressure.”
  11. In other words, morning wood is a welcome occurrence — a reminder first thing in the morning that you’re in good sexual health. Maybe that’s the reason we have such a soft spot in our hearts for the term (pun, of course, completely intended). It’s the gentle, harmless and accepted way of referring to a boner, which is both a juvenile and oddly ugly word for the male erection. The term is certainly used almost everywhere by almost everyone. From the name of alt-rock bands to American Porter to a night of English stand-up comedy at Gustoes Beer House in Bangalore — and pretty much everything in between. The only usage we can’t endorse: Dressing up as morning wood for Halloween.