Although I don't remember much of my chemistry study, I still remeber that we don't convert material into energy unless we are splitting the atom. Digestion is only a chemical reaction.

A chemical reaction may either require or produce an amount of energy. This is bonding energy inside the molecules and the total weight of materials before the reaction should be equal to the total weight of materials after the reaction. I may be wrong. So please feel free to correct me.

If you believe so, then the only explanation for losing 3-4 lbs during sleep is that some material which weighs 3-4 lbs has left your body. This must have happened through breath, sweat, urine or feces.

We lose some carbon in the form of carbon dioxide in our breath which may contribute alittle to the weight loss. The rest, if not lost as urine or feces, must have been lost as sweat.

Sam