losing weight overnight

I sometimes weight myself right before I go to bed, and then in the
morning. The weird think I have noticed is that I can actally lose
1/2kg or more overnight. Yes, I may pee during that time, but surely
that does not account for the rest of it. I suppose it is do with
water retention, and some also goes as sweat, but it does seem a
staggering amount.

Is that normal? I am not concerned, just more surprised really. Oh, I
way about 71kg (for % purposes).