When it’s deprived of sleep, your own body will cause you to put on weight, due to a number of factors. The first concerns two hormones called leptin and ghrelin. Leptin tells your brain you feel full, so when levels are high, your appetite is blunted and you don’t want to eat. Ghrelin, in contrast, ramps up feelings of hunger, so when levels are high, you feel hungry and want to eat more. Generally, there’s a healthy balance between these two hormones and the opposing forces of hunger and food satisfaction they control.
A lack of sleep distorts this balance, though. Take a group of healthy, fit individuals and limit them to four to six hours of sleep a night for several nights (as countless research studies have done) and you see a striking rise in levels of ghrelin, leaving participants constantly feeling hungry. At the same time, levels of leptin decrease, unleashing an extraordinary appetite.