How sleeping less makes you gain
If you’re what researchers call a short sleeper (measured by how long you sleep each night—5.5 to 6 hours or less qualifies you), you’ll have trouble losing weight, no doubt about it. In a 7-year study of 7,022 middle-aged people, Finnish researchers found that women who reported sleep problems were more likely to experience a major weight gain (defined as Sleep less, burn fewer calories
In a study published in the American Journal of Clinical Nutrition, researchers had a group of men sleep for 12 hours a night but didn’t allow them to sleep the next night, and then had them eat a big buffet the following morning. The researchers then measured the subjects’ energy expenditure—the calories you burn just by being. When the men were sleep deprived, their general energy expenditure was 5% less than it was when they got a good night’s sleep, and their post-meal energy expenditure was 20% less.

It’ll lead to snacking
According to research presented at the American Heart Association’s 2011 Scientific Sessions, women who got only 4 hours of sleep at night ate 329 additional calories the next day than they did after they slept nine hours. (Men ate 263 calories more.) In another study published in the American Journal of Clinical Nutrition, 11 volunteers spent two weeks at a sleep center on two occasions. During one period, they slept 5.5 hours a night, and during the other, they slept 8.5 hours. When the subjects were sleep-deprived, they increased their nighttime snacking and were more likely to choose high-carbohydrate snacks.

Your cravings go haywire
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. 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. 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.

Your body won’t let go of fat
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 five 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, but if they didn’t sleep enough, ghrelin increased. Since ghrelin also promotes the retention of fat, researchers theorize that a lack of sleep explains why the nonsleepers held onto body fat. This happens because the diet-unfriendly hormone reduces the number of calories you burn and increases glucose production.

You’ll 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 two hours to raid the fridge.