A man's penis becomes erect in response to sexual arousal and excitement, but it can also become erect as a result of deep relaxation as during REM-sleep (when most dreaming occurs). During deep relaxation specific neuro reflexes cause the smooth muscles in the penis to relax, thus giving him a (mostly massive) hard-on. Of course, night-time erections may also be caused by erotic thoughts or dreams. Most men have several erections each night. They are very important for oxygenating the corpus cavernosum.
