While asleep our metabolic rate increases such that we lose more than three times the amount of weight than if we are awake (awake but lying dormant, of course): 1.9g/min compared to 0.6g/min.
This increase in ‘caloric expenditure’ is not yet fully understood, but there are a number of reasons why we may lose more weight while asleep than awake:
We know that in rapid eye sleep (REM), in which we spend roughly 25% of our total sleep time, the brain’s metabolic rate (the rate at which it consumes energy) is very high, even more than while awake. And while one’s body temperature drops while sleeping, during REM it increases, and this too may cause increased caloric expenditure.
This is in addition to “changes in the hormones which govern hunger and satiety, leptin and ghrelin”.