When one is asleep, they always get "morning wood." During your sleep cycle, you get sexually aroused. Everyone does. The only exceptions to that rule are those with diseases that prevent arousal for medical reasons as opposed to psychological reasons. If you're worried about it, get a roll of stamps, wrap them around your penis before you sleep, and see if they're broken in the morning. If they are, then you have a psych problem, if not, then it is physical.