Researchers have found that your risk of weight gain can be influenced almost as much by your sleep as by your eating habits. Eve Van Cauter, PhD, at the University of Chicago, first hypothesized this was because the sleepless were overeating during those long stretches of night. To test the hypothesis, she recruited a group of young men to spend four nights in her lab.

For two nights the men were allowed to sleep only four hours, and for two nights their rest period was 10 hours. Importantly, two hormones that regulate appetite changed radically when the subjects slept less: Leptin, which signals the brain to feel full and stop eating, decreased by 18 percent, and ghrelin, the hunger hormone, increased by 28 percent.

For the first time, Van Cauter was able to establish that sleep deficits are capable of triggering a damaging hormone cascade. These hormonal changes, Van Cauter observed, suggest that if the subjects had unlimited access to food — which they did not — they would have eaten more and gained weight. 

Since her work was published in 2004, countless studies have provided support. At Columbia University, researchers reported that those who regularly slept just four hours were 73 percent more likely to become obese than those sleeping between seven and nine hours. (Even people sleeping a more respectable six hours were 23 percent more likely to become obese.)

Researchers from Uppsala University in Sweden used MRIs to show that sleep loss triggered the area of the brain associated with hunger and the desire to eat. And Van Cauter ultimately concluded that sleep restriction disrupts the daily drop-off of the damaging stress hormone, cortisol, which should be at its lowest levels right before bedtime and which is implicated in weight gain.