Hi, Thanks for writing in. At 16 years you are in your adolescence. This is a time when your body is transforming to that of an adult. Your testes are increasing in size and have started producing sperms as in every adult male. The sperms are manufactured in the testes as a result of hormonal developments. Each day a small amount of semen is made. When enough semen is there in the testes, it overflows in the form of nightfall. This is a normal process and not some thing to worry about. You can read about this under adult male sexual development online and also ask your biology teacher. The way to avoid nightfall is to masturbate once in 2 - 3 weeks. This will not allow the semen to overflow. Please note that masturbation done occasionally is normal and not a bad habit.