There has recently been some interesting research as to how weight is lost as we sleep. A researcher found that while subjects slept, they individually lost about 1/4 pound per hour or 2 pounds per night. If awake, but lying in bed for 8 hours, they lost weight at only a third of that rate.
The theory is that elevated brain activity during REM sleep increases metabolism over resting metabolism while awake. Also, there has been discussion that people who are sleep deprived can find their appetites increased due to stress hormones.