For many, the ringing of the alarm clock in the morning can be an unpleasant sound to wake up to. Accompanied by this unpleasant sound is the highly inconvenient phenomenon grown men, little boys, and even males in utero all experience: nocturnal penile tumescence (NPT), also known as “morning wood.” Unlike the Bermuda Triangle, this is not one of life’s unresolved mysteries, and ASAP Science’s “The Science of ‘Morning Wood” pulls back the covers on how and why morning erections occur.
The physiological reaction that affects half the world’s population happens when males are sound asleep. NPT, just like clitoral erections, has been associated with rapid eye movement (REM) sleep. During this stage, the brain begins to shut off some neurotransmitters in an effort to regulate the body and stop you from acting out your dreams. Norepinephrine, a neurotransmitter that happens to be involved with the control of erections, causes vasoconstriction of penis blood vessels, which actually prevent an erection.
source: medical daily