Is Masturbation A Sin?

I don't see the word or the subject mentioned anywhere in the Bible. This is a touchy subject, I know. Latest statistics says 90% of males masturbate and about 60% females. I know the Bible touches on marriage and sex is marriage only is right. Outside of marriage is adultery, a sin. But what about all the single people like me out there? Sexual release is, to me, a normal biological function. I think the monks don't do it, they have what is caused 'nocturnal emissions', I guess the nuns have something similar. The subject troubles me for I do not want to displease God if this is a sin. After I do it, I always repent sincerely and just claim that it is a 'normal biological function', and ask Him to forgive me on those grounds. Like to hear from others who have thoughts on this subject, and if you can give me Biblical scripture or references regarding it I would appreciate it. Thanks.(Please be serious with this, no silly or making fun replies, okay?)