Contrary to what many believe, waking up with your flag at full mast is not caused by urine buildup in the bladder. Morning erections are technically night-time erections (or Nocturnal Penile Tumescence) which happen three to five times per night.
They usually pop up (pun intended) during periods of Rapid Eye Movement (REM) sleep (when most dreaming occurs) and last around 30 minutes each.
Unrelated to dream content, they are both common and completely normal and men older than sixty years may even have them during non-REM sleep.
Likely a combination of testosterone levels and bedtime brain activity, these spontaneous woodies in the wee hours are caused by specific neuroreflexes that are stimulated during REM sleep.
Scientists believe that the nerves in the penis release nitrous oxide which dilates nearby blood vessels, increasing blood flow and leading to an erection.
Though you may find it surprising, this phenomenon occurs in other mammals. Women experience similar yet less visually noticeable REM-related engorgement of the clitoris and vagina as well.
Thought to be the body’s natural attempt to properly oxygenate the penile tissue, most guys will have nocturnal erections throughout their lifetime, and their frequency, along with one’s awareness of them, may vary.
The penis may stay erect for up to two hours nightly, and not all men who have erections during the night wake up with one in the morning.
So despite the name, the common experience of “morning wood” has nothing to do with time of day, but rather the flag raiser’s stage of sleep.