Did you know that you lose weight overnight?? Sure, when you exercise, you lose weight because you burn off fat. But in your sleep you lose water weight. Every time you breathe out, you lose water vapor as lots of teeny droplets. You also sweat out water through your skin. Grownups can lose more than a pound overnight, which is 2 cups of water. Try weighing yourself right before bed (after peeing) and again when you wake up (before peeing), and see what you find out!