During REM (dream) sleep, males usually get erections and female genitals swell and lubricate. Since humans have a much greater proportion of REM sleep toward the morning, men frequently wake up with REM-related erections. Morning erections are not necessarily a sign of sexual arousal, and will often feel a bit different to a man than an erection he gets when he wants to have sex. Also, males might awaken with elevated levels of testosterone in their blood, which might contribute to morning erections. AKA morning glory.