Lost Sleep=Gained Pounds: Running a sleep deficit can lead to a surplus around the middle.

Researchers at Penn State University looked at studies of people who reported getting between four and six hours of sleep a night and found that the lack of sleep altered hormones in a way that leads to weight gain.

The study, in the Journal of the Academy of Nutrition and Dietetics found that levels of the hormone ghrelin were higher than normal among the sleep deprived, and levels of the hormone leptin were lower than normal.

Increased levels of ghrelin is associated with increased appetite, while leptin is associated with feeling satiated.

Being up more hours also offers more hours to eat and the researchers found that when people eat tired, they tend to eat food that's higher in fat and/or sugar — leading to more weight gain.

And there's more: Even people who manage to cut calories while not getting enough sleep, their sleep-deprived state appears to alter the way their bodies lose weight. Rather than losing fat, as rested dieters would, the sleep-deprived lose less fat and more lean mass. So cruel!