With an ever-increasing number of studies finding a direct connection between sleep deprivation and weight gain, it's difficult to deny the cause-and-effect relationship. People who get at least seven hours of sleep per night tend to have less body fat than people who don't. There are, of course, other factors involved in determining who becomes overweight and who doesn't, like food intake, exercise and genes. But sleep is a more integral of the process than most people realize. In a study involving 9,000 people between 1982 and 1984 (NHANES I), researchers found that people who averaged six hours of sleep per night were 27 percent more likely to be overweight than their seven-to-nine hour counterparts; and those averaging five hours of sleep per night were 73 percent more likely to be overweight.

Many people who are sleep deprived don't even know it. Lots of us think there's quite a bit of give in how much sleep a person needs to be healthy and well functioning, but most researchers disagree, putting seven hours as the minimum for all except the very young and the very old. Besides straight numbers, there are a couple of ways to tell if you're sleep deprived, including:

  • Are you typically drowsy during a good portion of the day, especially the morning?
  • Are you falling asleep at night in a couple of minutes?

Most non-sleep-deprived people take about 15 minutes to fall asleep at night. Chronic sleepiness and a nearly-instant state of sleep when you get into bed are good indicators that you're not getting enough sleep.

If you are sleep deprived, there are some obvious tie-ins to obesity, like chronic sleepiness making physical activity unlikely. But there are also a number of things going on in your body that could contribute to weight gain. In scientific studies, the most commonly cited effects of sleep deprivation are hormonal disturbances, specifically involving the hormones leptin and ghrelin.

When you don't get enough sleep, your body has too little leptin and too much ghrelin.