An involuntary male orgasm during sleep is properly called a nocturnal emission. It is usually referred to as a wet dream

The testis begin to produce sperm during puberty, although semen production may well be established before that of sperm making it quite possible for some boys to ejaculate before they are fertile. The semen and sperm build up and seek release. If the man or boy does not masturbate or share sex he is likely to find that ejaculation has taken place while he sleeps.

Many, but by no means all, nocturnal emissions are accompanied by erotic dreams, passive stimulation by bedding etc or active stimulation during sleep. Even having a full bladder can trigger a nocturnal emission.

Nocturnal emission is not a sign of any physical or emotional problem. It is a normal reaction to the body's development. Likewise, the absence of nocturnal emission is not a sign of any problem, although if an adult male presented at his doctor's, concerned his testosterone levels may be low, a lifetime lack of any nocturnal emissions is one of the things that would be likely to be taken into account.

Although men of all ages experience nocturnal emission,  is more common during puberty than in adulthood, due to both the large surges of testosterone present in the body during adolescence and the likelihood that a person of this age is less likely to be releasing semen in other ways.
It is believed that about 85% of men will experience at least one nocturnal emission during their lifetime. Most nocturnal emissions happen to young men between twelve and eighteen years of age, tapering off as hormone levels stabilise and/or masturbation and sharing sex take a more prominent role.

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