The Science of 'Morning Wood'
Morning wood: It affects men of all ages and even happens when they're still in utero. You know, the ability to pitch a tent in the morning without any actual camping skills? Now that we've got the one obligatory joke out of the way we can get to the hard facts of nocturnal penile tumescence, otherwise known as morning wood.
It turns out, morning erections are part of the normal sleep cycle and occur multiple times throughout the night. The mind and body go through stages of sleep from non-REM (rapid eye movement) stages to full on REM deep sleep, which occurs about four or five times a night.
As you enter REM sleep, not only do dreams occur, but a few physiological changes take place as well. Namely, your brain begins to shut off some neurotransmitters in an effort to regulate your body, and in some cases, stop you from acting out your dreams. One of these neurotransmitters, norepinephrine, happens to be involved in the control of erections. Specifically, it causes a vasoconstriction of penis blood vessels actively preventing an erection. It's like a stop sign to blood flow.
But as you enter REM sleep, norepinephrine decreases and testosterone-related actions are now able to take place. This leads to vasodilation (increased blood flow) to the blood vessels. Ultimately, causing an erection. Why is this important? Well, similar to muscle and other tissue at night, this extra blood increases oxygenation serving as a system of repair and helping to maintain functionality.
But why do men often wake up to this pleasant surprise?
Healthy men typically have anywhere from three to five erections during the night, lasting about 25 to 25 minutes at a time. Which makes sense, since people enter REM sleep multiple times in one night. And when we're waking up, our body is still coming out of REM sleep. As a result, we also experience remnants of this sleep stage, which can include an erection.
There's also evidence that a full bladder can contribute to morning wood. The increased bladder size for the night stimulates a region of the spinal cord, which can cause a reflex erection. The physiological benefit of this is to prevent you from urinating in your sleep. But most men can attest to the difficulty proposed by this conundrum.