Nocturnal Emission

Involuntary discharge of semen during sleep; also known as nightfall or wet dreams.

Persons most commonly affected: Adolescents and adult men
Organ or part of body involved: Male reproductive system.
Symptoms and indications: May include restlessness, insomnia, mild night sweats, difficult urination, and mild emotional disturbances. 
Causes and risk factors: They are more common in males who masturbate less often.