Another thing to remember is to eat and drink enough before going to bed, especially in winter. If you don't take in enough energy before going to bed, you may wake up cold in the middle of the night. Staying warm burns a lot of calories, even if you aren't doing anything else other than laying in your bag. A snack such as chocolate or some other high calorie food will make a big difference.

btw, Michael Phelps was eating about 14,000 calories a day, which is close to what most people eat in a week.