How am I able to lose weight in my sleep?

I've noticed I'm waking up in the morning substantially lighter than when I go to bed at night (last night I weighed 171.5 lbs and this morning I weighed 165). My boss suggested we burn energy in our sleep but "burning energy" is simply a chemical process and you should have the same amount of "matter" coming out as going in. I also understand that I sweat in my sleep (and get up to pee once in a while) but 1 gallon of water weighs roughly 8 pounds and there's no way I'm peeing and sweating that much (the human bladder holds roughly 300 ml to 700 ml of liquid). I also have not *ahem* done "twosies" in between weighings. Where is the weight going?