This is probably the biggest revelation about the connection between sleep and weight loss—and the biggest challenge for you if you're not getting at least 7 solid hours of sleep each night. Sleeping too little impacts your hormone levels in ways that can undermine the efforts of even the most determined dieter. That's because insufficient sleep raises the levels of ghrelin, the hormone that tells you to eat. When it comes to weight gain and loss, this hormone plays a leading role.
Ghrelin's job is to boost your appetite, increase fat production, and make your body grow—all of which are fine things if you're a lanky 12-year-old. But once you're in your thirties, ghrelin's effects can seem pretty darned undesirable. It's a cinch to figure out why this hormone is the last thing a dieter needs to have circulating in excess.
Lack of sleep also lowers levels of leptin, the hormone that says, "I'm full; put the fork down." Leptin's levels run high during the night, which tells your body while you're sleeping that you don't need to eat. Its levels drop during the day, when you need food as energy. So high leptin levels keep hunger at bay. In studies, for example, mice lost weight because leptin made them eat less and exercise more: the holy grail of dieting. But if you don't get enough sleep, your leptin levels plummet.
So after even one night of too little sleep, leptin and ghrelin become dietary gremlins bent on diet-wrecking mischief. The lower leptin levels mean that you still feel hungry after you eat. And ghrelin, for its part, magnifies the problem by stimulating your appetite, setting the stage for a day of unsatisfying, high-cal feasting after a restless night.
In the Wisconsin Sleep Cohort Study of more than 1,000 people, researchers found that people who got 5 hours of sleep a night had 15.5 percent lower leptin levels and 14.9 percent higher levels of ghrelin, compared with those who got 8 hours of sleep. Know what else the nonsleepers scored higher numbers in? BMI. So more ghrelin plus less leptin equals greater body mass index and weight gain.
In a study at the University of Chicago, researchers discovered that restricting the sleep of 12 healthy young men to 4 hours a night lowered their leptin levels by 18 percent. The men rated themselves as having a 24 percent increase in hunger.
Sleep Less, Hang On to Fat More
Lack of sleep may also affect the kind of weight you lose. In another study at the University of Chicago, researchers followed 10 overweight but healthy subjects who were placed on a balanced diet, then observed in two 14-day increments, one in which they got about 7.5 hours of sleep, and another in which they got 5 hours and 15 minutes.
During both periods, the subjects lost an average of 6.6 pounds. But when they got more sleep, they lost 3.1 pounds of fat, whereas during the short-sleep period, they lost only 1.3 pounds of fat. Those who got more sleep reported less hunger, which makes sense: When they got enough sleep, their ghrelin levels stayed the same. On the 5-hour nights, their ghrelin levels rose by 9 points.
Since ghrelin also promotes the retention of fat, researchers theorize that a lack of sleep explains why the nonsleepers held on to body fat. This happens because the diet-unfriendly hormone reduces the number of calories you burn off and increases glucose production.
Sleep Less, Have More Time to Eat
It hasn't been scientifically proven, but some experts believe that the 2 hours or more that we're no longer using to sleep is giving us another 2 hours to raid the fridge.
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