nocturnal emission 
noun
An involuntary ejaculation of semen during sleep.The Indian researchers found that many of the main sexual concerns of south Asian males are focused on semen loss-from masturbation, nocturnal emissions and other causes....
- It is normal also to not have them or to not realize you are having the nocturnal emission because the semen may dry before you wake up.
- In point of fact, though, we no longer believe that monks who have nocturnal emissions are victims of demonic succubae, nor are they morally culpable; yet this was a common belief once.