Semen production goes on in the testis for 24 hours and gets stored in seminal vesicles. Now imagine seminal vesicles as a water tank. 24 hours production at some time, will make tank full. Its storage capacity runs out. Now overflowing mechanisms take place like night falls and seminal discharge in urine. To avoid this u need to have sex or masturbate regularly.

Nocturnal emissions are very common and normal thing It occurs in males who acheive puberty. It is very common in males who have abstinence from masturbations and sexual intercourse There is nothing abnormal about it Nothing to worry Be relaxed