Morning wood - scientifically named Nocturnal Penile Tumescence (NPT) - is the random occurrence of an erection that men can experience upon waking up in the morning. I'm sure most men have dealt with it at least once in their lifetime, but why and how does it happen?
A popular reason for why morning wood occurs is because erections usually happen when a guy is relaxed, which is typical for someone who has just woken up in the morning. It turns out that erections are also common during the Rapid Eye Movement (REM) cycle of sleep, which is the deepest level of sleep; they can occur 3-5 times a night. Another factor that may induce an erection is a full bladder. This is because the bladder fills up during the night and can put pressure on the spinal cord which may trigger an erection.
From a medical standpoint, morning wood (and more specifically the lack of morning wood) is helpful in diagnosing erectile disfunction. Doctors can use a device to monitor a patient during sleep and detect if erections are occurring. If none appear, then it can be ascertained that the E.D. the patient is experiencing is due to one's body and not from psychological reasons.
* Fun Fact: In the U.K. morning wood is referred to as dawn horn