We all know that weight loss (and gain!) is about much more than food: you’ve got to factor in exercise, mental attitude and environment as well. But fewer of us know that getting enough sleep is also a crucial element in any sensible weight-loss or weight-maintenance program.

In fact, studies consistently show a link between too little sleep and increased body weight in both adults and kids, according to the nationwide organization TOPS (Taking Off Pounds Sensibly). Why? Nick Yphantides, M.D., spokesman for TOPS, says if you’re up too late, you’re liable to eat too much—and get the next day off to a bad start.

“Falling asleep with a full stomach,” Yphantides says, “means you’re less likely to eat breakfast, which is an essential part of a healthy lifestyle. Every study of successful long-term weight loss shows that the eating plan includes a healthy breakfast.” And breakfast isn’t all you’re likely to skip. When you’re tired from staying up late the night before, you don’t have much motivation to exercise.

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Too little sleep also alters hormones that affect your appetite. “Physiologically, when a body isn’t rested, it kicks into survival mode,” Yphantides explains. “Stress hormones are generated.” That raises the production of an appetite-stimulating hormone known as ghrelin and lowers the production of leptin, a hormone that suppresses your appetite.

And as if that isn’t enough reason to make you bounce into bed at 10 p.m., consider this: Lack of sleep can have a negative impact on your immune system, your memory, your concentration and your blood pressure. It’s been linked to depression as well.