If you sleep in the same bed as someone with a penis, your partner's boner poking you in the back in the morning is like a natural alarm clock: inevitable, not always welcome, and hard to snooze. And it's not just in the morning: Men get three to five erections during one night of sleep, and each one can last between 20 and 30 minutes. But does that mean that each of those times your partner gets hard they're turned on and want to have sex? Not exactly, and most people can't help that they randomly get boners in the middle of the night.
The proper term for "morning wood," or night boners, is "nocturnal penile tumescence" (NPT). Nocturnal erections seem to follow a man's sleep cycle, and usually happen during the REM phase of sleep, says Aleece Fosnight, MSPAS, PA-C, a urology physician assistant and a sexual health counselor. "It doesn't mean that he is aroused or had a sexual dream or fantasy, but rather [it's] the body's way of ensuring the penile tissue remains healthy," Fosnight says.
So, if they're not aroused, why exactly do people get full-fledged boners? There's a neurotransmitter called norepinephrine, and it's responsible for stopping blood flow from the penis, among other things, Fosnight says. "When your body goes into REM sleep, norepinephrine actually drops, causing a rush of blood flow into the penis," she says. "The way that 'morning wood' happens is when you wake up during one of those REM cycles when the penis is fuller." This might not happen every morning, because, technically, people with penises have to be experiencing REM sleep to wake up with a boner, and you usually don't wake up during REM, because it's the deep sleep phase. But still, morning wood is incredibly common, Fosnight says.
Some experts also say that when people with penises have a full bladder, there's a mechanical pressure that their brain interprets as pleasurable sexual arousal, and causes an erection, says Laurie Watson, LMFT, certified sex therapist. Either way, when a person wakes up with a boner, there's a good chance they weren't aroused before. (Of course, that doesn't mean they can't become aroused once they realize they have a boner.) And this isn't just biology's way of messing with us; it could be evolutionary, Fosnight says.
"Most speculate that [NPT] helps to keep the penis healthy by promoting oxygen-rich blood flowing into those tissues," Fosnight says, adding that NPT could also possibly prevent erectile dysfunction, or it could just be a sign that the penis is working normally. "Erections that occur during sleep are completely normal and happen nightly throughout a man's life and are not caused by sexual stimulation," she says.
And even though these boners may wake up sleeping partners in the middle of the night, NPT is considered beneficial from a sexual health perspective, too. "NPT is a wonderful thing, because it shows that a man is capable of achieving an erection organically," says Eric Garrison, a clinical sexologist. "If he is incapable of achieving an erection with a partner, though he experiences NPT, then we would assume that there is an emotional cause for his erectile concerns."
So, the next time your partner bumps you with their hard penis, they're not necessarily trying to have sex, but you can consider it an opportunity to ask, "You up?"
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