I too have the same thing where i am always 36 lbs lighter in the morning.

What we have to remember is we are not completley inactive during the night - we are moving and fidgiting (and more if you are lucky, he, he!). We also breath, wish of course contains water in our breath. Some of this apparent weight loss could be from using up water during the 8 or so hours we are asleep. After all most of us wake up needing a drink don't we?

Or we are whisked away in our sleep by the whoosh fairy and force-fed magic carb-free bars of dairy milk and ferreo roche with no effect whatsoever.

Thats my 2 cents