Alexis on the Sexes: Pee hard

ALEXIS MCKINNIS | Updated 12/9/2013

Is morning wood just a full-bladder thing?

Note: Alexis has the week off, so here’s an oldie-but-goodie.


Q: My partner has a woman friend who seems to be wise. When she wakes up in the morning and her husband’s private parts are at full attention, she remarks with a foreign accent: “I don’t trust this.” Her underlying belief is that this is a “restroom arousal” rather than a sexual one. Is this woman on to something? Todd


A: Your partner’s friend is partially right, or rather she might be right a small percentage of the time. The vast majority of occurrences of nocturnal penile tumescence (“morning wood”) happen during REM sleep. That much scientists do know, although the definitive reason is still a little unclear. There is a popular hypothesis, though: Certain neurotransmitters that keep erections at bay when you’re awake take a break during REM sleep, letting your testosterone take over for a while. Whoomp, there it is: a boner. It’s a perfectly good scientific explanation, one that might even be explored someday to discover the reason for nocturnal arousal in both sexes.

The full-bladder theory has some merit, too, though it doesn’t seem to be mentioned nearly as much as the aforementioned neurotransmitters (at least, not in the scholarly papers on popping chubbers that I’ve been reading). Actually, there are two theories as to why a full bladder may cause morning wood. Bladder Theory No. 1 is that it creates a small amount of pressure on nerves in the vertebral canal of the sacrum (the triangle-shaped bone wedged in the middle of your pelvis). Unintentional stimulation of these nerves is believed to sometimes cause erections, but it’s usually so faint that any other stimuli you experience while you’re awake distract you enough that you don’t accidentally pitch a pants tent. Since you’re not distracted when you’re sleeping, those nerves are free to do what they want, and if your bladder is full enough to tickle them, they might just make a woody.

Keep in mind that a man’s bladder would have to be pretty full in order to deliver the amount of pressure required to stimulate those sacral nerves, or, as is suggested in Bladder Theory No. 2, to push down on the prostate. (I found even less material on the prostate theory, rendering it even more of a “what the hell, it plays” hypothesis.) The idea is simply that a full bladder can cause enough pressure on the prostate to create an erection. The prostate, located at the base of the bladder, is charged with the duty of contributing to and storing semen. It also houses smooth muscles that push semen out in a projectile fashion — that little miracle of nature we call ejaculation. Massaging the prostate during sexual stimulation — as any man who’s had a finger up his butt will attest — can make orgasms stronger because of this function.

Now, go to the Wikipedia page titled “Erection” and search the page for the word “prostate.” You’ll notice it isn’t mentioned anywhere, which gives reason to doubt the prostate theory. The only weight it seems to have is that the sacral nerves that signal ejaculation are basically the same ones that signal erection, which, if you’re still following along, you’ll notice takes us back to Bladder Theory No. 1.

The human body is a wonder and its functions are fascinating if not downright baffling. The one thing your partner’s friend can trust about her husband’s morning wood is that, for whatever reason, it’s going to be there. Testosterone production decreases slightly around age 40, meaning she may not see it as often after that, but as long as the hormones are pumping, her man will get it up. Asleep or not.

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