Answers (3)
Depending on how old you are, a man at the peak of puberty will produce enormous amounts of semen for several months. Most guys masturbate heavily during that period. Some few are lucky to have a female to handle the problem. If you don't relieve the excess somehow, it will leak out while you are sleeping. This is normal and will stop after a while.
What you're having are commonly referred to as "wet dreams" and you don't lose your all your semen. When having sex or masturbating, after ejaculation it takes men differing amounts of time to "recharge, from only a few minutes for some and maybe a little longer for others. When you're older you'll wish you still could have these nocturnal emissions. So don't worry, happens to all us men. And when you are overcharged, it is preferrable to have a real girlfriend available.