Several studies show some pretty strong connections between lack of sleep and weight gain. One such study, the Mayo Clinic reports, showed that women who got less than six hours of sleep (or more than nine) were more likely to pack on a whopping 11 pounds (5 kilograms) than females nodding off for seven hours nightly. Another study on the subject found men who lacked adequate Z's took in more calories each day and preferred high-calorie foods.
So what's sleep got to do with it? One somewhat obvious connection is the longer you're awake, the more time you have to get hungry and eat. Some posit that if you're tired from a lack of sleep, you're less likely to be active and burn calories. Others say the amount of sleep you get each night is tied to the hormones regulating hunger; if you mess them up, your appetite becomes heightened [source: Hensrud]. The easy answer is to simply get a good night's sleep — seven or eight hours — every night. And why not? It sure feels great!