From news.bbc.co.uk:

Too little sleep may increase the chance of becoming obese, a study suggests.

People burn fewer calories asleep, so it might seem to be counterintuitive that more sleep prevents weight gain.

But it was found people who slept four hours or less per night were 73% more likely to be obese, possibly because of effects on appetite hormones.

The Columbia University study was presented to the North American Association for the Study of Obesity annual meeting in Las Vegas.

Also: Sleep may cut childhood obesity