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Nocturnal Penile Tumescence

Nocturnal Penile TumescenceMen have several erections while they sleep at night.

The clinical name for these erections is nocturnal penile tumescence (NPT). They happen to males of all ages, even babies, but they have nothing to do with sexual stimulation.

So why do they happen? Scientists aren’t completely sure, but NPT seems to be affected by sleep cycles.


Who knew? There's a science of "morning wood." Click here to tweet.


Humans go back and forth between deep sleep and rapid eye movement (REM) sleep several times during the night. REM sleep is lighter sleep and is the cycle when dreams occur.



During REM sleep, the brain produces smaller amounts of certain neurotransmitters. One is norepinephrine, which plays a role in erections. Norepinephrine’s job is to stop blood from flowing to the penis to prevent erections at inappropriate times.

But during REM sleep, levels of norepinephrine decrease, allowing testosterone to trigger more blood flow to the penis and, consequently, an erection. This cycle might occur four or five times a night.

Men often wake up with erections because they are just finishing a phase of REM sleep.

Some men might feel a little embarrassed by NPT, but these erections are natural and normal. They help keep the penis in working order. In fact, men who don’t have NPT should see their doctor to make sure everything is all right.

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ASAP Science

“The Science of ‘Morning Wood’”

(Video. November 14, 2012)

International Society for Sexual Medicine

“What is nocturnal penile tumescence (NPT)?”

Medical Daily

Borreli, Lizette

“The Science Of Morning Wood: Electrical Impulses In Brain Increase Blood Flow To Penis In REM Sleep”

(October 16, 2014)