Why do males experience morning wood?

I'm of course talking about nocturnal penile tumescence (NPT), often experienced as a waking (and perhaps unexplained) erection. However, the stiffening in question is not the work of devilish succubi or experimenting aliens (both sometimes share the blame). Rather, a healthy male experiences up to five of these a night as part of his natural REM sleep.

In fact, NPTs are so normal that doctors test for them when diagnosing erectile dysfunction. If these erections occur on their own during the night, then the patient's problem is likely psychological rather than physiological.

But why do they happen at all? Since they occur during the REM cycle, there's always the potential for a sexually-charged dream tie in, but it's not necessary. The main NPT theories actually have nothing to do with dreams. Rather, the body may be oxygenating the penile tissue as a sort of maintenance procedure -- and the stimulant for this is thought to be the release of nitric oxide by nerve fibers in the penis.

Oh, and then there's the whole full-bladder angle. The two often occur together and scientists have taken note. So some theories view morning wood as the body's anti-bed wetting measure, or perhaps a wake-up call to the sleeper himself.

So in other words, gentlemen, a little morning wood (or even a lot of it) is nothing to worry about.

About the Author: Robert Lamb spent his childhood reading books and staring into the woods — first in Newfoundland, Canada and then in rural Tennessee. There was also a long stretch in which he was terrified of alien abduction. He earned a degree in creative writing. He taught high school and then attended journalism school. He wrote for the smallest of small-town newspapers before finally becoming a full-time science writer and podcaster. He’s currently a senior writer at HowStuffWorks and has co-hosted the science podcast Stuff to Blow Your Mind since its inception in 2010. In his spare time, he enjoys traveling with his wife Bonnie, discussing dinosaurs with his son Bastian and crafting the occasional work of fiction.