For the study, researchers from the University of Chicago Medicine and the Medical College of Wisconsin had nine young, slim, healthy adults spend two six-day stints in a sleep lab. During the first session, they slept 8.5 hours a night—and during the next, they slept just 4.5 hours a night.
The researchers found that the participants’ peak levels of 2-AG—a molecule that influences how much pleasure you get from eating—were 15 percent higher if the subjects were sleep deprived. The molecule is something called an endocannabinoid, which can trigger cravings for calorie-dense foods similar to the munchies caused by marijuana’s cannabinoids, says study author Erin Hanlon, PhD, research associate at the University of Chicago’s section of endocrinology, diabetes, and metabolism.
“Sleep deficiency influences hedonic mechanisms so that highly palatable or high-reward foods are preferred and consumed,” she says. Translation: Skimp on the shuteye, and you’ll want to load up on high-fat and high-sugar foods the next day.
Previous research shows that sleep deprivation contributes to increased appetite, weight gain, and type 2 diabetes. While sleeping fewer than six hours a night spikes hunger by influencing levels of appetite-regulating hormones leptin and ghrelin, this study explains why junk food might be so hard to resist after a sleep-deprived night—even if you aren’t feeling hungry. Researchers aren’t sure why a lack of shuteye raises 2-AG levels, but they say it could be your body’s way of ensuring you increase your energy (aka calorie) intake to perk up after a less-than-restful night.
According to previous research published in the journal SLEEP, nine hours a night is the optimal amount of sleep when it comes to your waistline. Still, everyone’s exact target number is different, says Nathaniel Watson, MD, MS, co-director of the University of Washington Medicine Sleep Center. Want to find yours? The next chance you get, go to bed when you feel tired and wake up in the morning sans alarm. After a few days of this, calculate the average number of hours you snoozed each night, says Watson. That’s how many you need to stay healthy—and slim.