It effects grown men, little boys and even happens to males still in utero.
You know, the ability to "pitch a tent" in the morning, without any actual camping skills.
Now that we 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 throughout the night.
The mind and body go through stages of sleep, from non-REM stages, which gradually become deep sleep finally followed by REM or "Rapid Eye Movement" sleep which is actually fairly shallow sleep.
This cycle occurs between 4 or 5 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 neurotranmitters, norepinephrine, happens to be involved with the control of erections.
Specifically it causes vasoconstriction of penis blood vessels, actively preventing an erection.
It is like a stop sign for blood flow. But as you enter REM sleep, norepinephrine decreases and testosterone related actions are now able to take place.
This leads to vasodilation, or 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 we often wake up to this pleasent surprise?
Well, we often wake up just out of REM sleep, and as a result see the remnants of this sleep stage.
There is also evidence that a full bladder can contribute to morning wood.
The increased baldder 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 after waking in the morning.
Rise and shine.
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