Diabetes? Kidney problems?
Anyway, you don't just pee out water, you also sweat and breathe. But 6-8 pounds is quite a bit. So tell us how much you drink in a day, how much you eat in a day, if you by any chance consume alcohol or any other diuretics. Is it like this every night or just some nights? How long has it been like this?
You can PM me (or answer here) and I'll try to find the problem or go to a doctor and (if he doesn't suggest it) ask him to test your urine and your blood.
It's not pleasant, but measuring how much you pee per night would be of diagnostic value. If you do it, weigh yourself before and after that night as well.