Most men wake up in the morning with an erection, or at least a large amount of the time they do. It is also often accompanied by a dire need to use the bathroom. This is sometimes referred to as 'morning wood' but the scientific term is nocturnal penile tumescence. Although many reputable scientific theories have formed to explain nocturnal tumescence, none have been proven.

Many non-scientist men accredit this to urine backup, which is understandable, but unlikely from a scientific viewpoint. Some simply believe it's because men wake up with an erection because they have had sexual dreams. Indeed, this could explain why a man has to use the bathroom after, to release male liquids.

According to most scientists, men have several erections during sleep because the penis needs oxygenated blood, which is gets during erections. But through much of the day, this is controlled by men, who usually learn to carefully control their erections.

During the night, though erection isn't controlled, and because of it, the penis is oxygenated frequently, several times at night sometimes lasting two hours.

Becauses of nocturnal tumescence, doctors can easily test to see why a man is impotent. There are two causes of impotence - psychological and vascular reasons. A device can be hooked up to the penis to see if it becomes erect at night. Because control and pshycological reasons aren't present during sleep¹, they would have erections during the night. If a man is not having erections, it is a vascular problem. This test is called the Nocturnal Penile Tumescence Monitoring.