Sometimes, erections during sleep lead to nocturnal emission or "wet dreams." During a wet dream, boys ejaculate excess sperm. Wet dreams can happen as often as one to three times per week, but some men never experience a wet dream in their lives.
Typically, men who ejaculate on a regular basis through sex or masturbation have fewer wet dreams than men who do not. Women experience wet dreams as well, though it is not as messy as wet dreams in men.