When you don't get your ZZZs, it can make it harder to lose weight. Your metabolism may slow and you won't burn calories as fast as you'd like. You may also have less energy when you don't get enough sleep. That makes it harder to exercise. When you're tired, you're more likely to make poor diet choices, like choosing sweets over fruit. In a recent study, people who didn't get enough sleep ate about 300 more calories per day than those who got more rest.