Morning wood, or, as your doctor would prefer you call it, nocturnal penile tumescence (NPT), is typical in young guys. You’ve probably woken up next to a drowsy boyfriend who somehow has a rock-hard penis and wondered why he’s so aroused. “It’s a pee-boner,” he insists, citing that age-old claim that he just has a boner because he has to pee so bad. Well, he’s wrong.
Joseph Alukal, M.D., assistant professor of Urology and Obstetrics and Gynecology at NYU Langone Medical Center, explains, “It’s not directly related to that.” But, weirdly enough, although researchers can tell us what doesn’t cause morning wood, what does cause it is still kind of a mystery. “The penis is a very strange organ in terms of its job and the way it traps blood inside it,” says Dr. Alukal. Here are some reasons doctors think guys’ bodies wake up ready to bone.
1. His penis is going rogue because his brain isn’t paying attention. According to a study by the Society for Endocrinology, it could actually be that as certain parts of the brain shut down during REM sleep, his brain is no longer keeping his penis in check. Whereas normally the brain is regulating the penis and making sure erections only occur when needed, during REM the penis is free to do whatever it wants. And what it wants to do is get hard.
2. It’s just his hormones fluctuating.“In the morning, it’s going to be the highest testosterone level he’ll have all day,” according to Dr. Alukal, noting that morning boners tend to stop when men reach their 40s and 50s and their testosterone levels dip. So it could just be that he’s at his manliest (and hardest) in the morning.
3. It’s how his penis “exercises.” NPT can actually be a way the body conditions the penis to keep blood in it for long periods of time. “There’s no other place in your body that has to trap blood for an extended period of time,” says Dr. Alukal. “For example, if we took a tourniquet and tied it around your foot for a half hour, we might have some problems afterward. But your penis is expected to trap blood for a half hour or an hour and be fine afterward. This could be part of the programming that reminds the penis, ‘This is how you do your job.'” Think of it like weight training. If it weren’t for nighttime boners, it’s entirely possible guys wouldn’t be able to keep their boners for more than a few minutes at a time without it becoming incredibly painful. But Dr. Alukal is quick to point out that this is just a theory, and there’s no data to back it up.
4. He rubbed up against something in his sleep and subconsciously got horny. Just because a guy is waking up with a boner, it doesn’t necessarily mean he’s been dreaming about Jennifer Lawrence. The brain is still receptive to physical stimulation when its asleep, so sleeping close his partner can absolutely set it off. “Your brain can still feel the stimulation and say, ‘Hey, wait a second, this guy needs an erection,'” says Dr. Alukal. So before you get out your dream analysis journal, consider that he might just be turned on by your hot bod.
5. And it (probably) isn’t for bladder control. This last one is worth mentioning because it’s a common misconception: the idea that a nighttime erection prevents guys from wetting the bed in their sleep (which is also known as “nocturnal enuresis” if you ever want to sound really smart when you’re talking about bedwetting) . Waking up with an erection and the urge to pee is what’s commonly referred to as a “pee boner.” Unfortunately, there’s very little scientific evidence to support this theory. Plenty of guys have probably experienced the symptoms of a pee boner when waking up, and probably jumped to this conclusion. There is some tangential evidence for NPT serving this function in that the spinal cord nerves that control reflex erections are also tied to the bladder. But the body has plenty of other systems in place to prevent bedwetting, so this doesn’t seem likely. Really, the kidney gets much better at regulating this function, so the idea that it’d need a guys’ body to pop a boner as a makeshift dam is unlikely. The fact that women also experience nocturnal clitoral tumescence also suggests that this is not related to bladder function.