The study, published in the Annals of Internal Medicine, finds that sleepless dieters and dieters who slept well lost the same amount of weight.
But there's a big difference: fat made up only a quarter of the weight loss in those who were sleep deprived. And they felt hungrier. In contrast, those who got a full night's sleep lost more than half their weight in fat.
"If your goal is to lose fat, skipping sleep is like poking sticks in your bicycle wheels," said Plamen Penev, an assistant professor at the University of Chicago and the study's director.
Dr. Penev says it appears that sleeplessness triggers an increase in ghrelin levels, a hormone that triggers hunger and reduces energy expenditure.
"For the first time, we have evidence that the amount of sleep makes a big difference on the results of dietary intervention," he said. "One should not ignore the way they sleep when they go on a diet."