The theory: You burn up the food you eat earlier in the day, while late-night calories sit in your system and turn into fat.
The reality: Calories can't tell time. "Your body digests and uses calories the same way morning, noon, and night," says Mary Flynn, Ph.D., a research dietitian at the Miriam Hospital, in Providence. They may sit around a little longer if you eat, then lie on the couch and watch Letterman, but when you move around the next day, your body will dip into its stores. That said, there are other solid reasons to avoid late-night snacking, not least of which is that snacks you grab when you're tired tend to be unhealthy ones.
The best advice: If you often unwind before bed with a bowl of ice cream or buttered popcorn, try cutting the snack out. The calories saved may be enough for you to lose a few pounds a year. If you're hungry, "eat something on the light side, like a piece of fruit or some cereal with milk," says Ellie Krieger, a registered dietitian and the author of The Food You Crave ($28, amazon.com). Night eaters tend to overeat (which leads to weight gain no matter when it's done) because often they've been skimping during the day and come home famished. Being so hungry that you grab whatever is at hand means you're more likely to make poor choices. "Don't go longer than about five hours without eating," says Jo Ann Hattner, a registered dietitian and a nutrition consultant in San Francisco. Just be careful to keep your meals and snacks small.