What causes a man to wake up with an erection? The short answer is good
blood flow and chemistry. It is a very healthy sign. It will probably
happen for most of your life.

Here is an article that explains it in more detail:
Morning wood, or, as your doctor would prefer you call it, nocturnal penile tumescence (NPT), is typical in every young guy. You've probably woken up next to a drowsy boyfriend who somehow has a rock-hard penis and wondered why he's so aroused. "It's a pee-boner," he insists, citing that age-old claim that he just has a boner because he has to pee so bad. Well, he's wrong.



