America is obsessed with diets, but sometimes gaining weight is more complex than calories in versus calories burned.

For example, weight gain may depend, at least in some cases, on how much sleep you get.

Sanjay Patel of Case Western Reserve University in Cleveland, Ohio, has conducted a 16 year study of nearly 70,000 women to test how sleep affects weight gain. He found that women who sleep five hours or less per night gain more weight on average than women who sleep seven hours or more.

The results varied, of course, but women who slept five hours or less per night were 32% more likely to really put on weight, on average 33 pounds or more.

Other studies have shown that not getting enough sleep can screw up the hormones that control appetite and make you feel hungrier. In this case the women who slept less actually ate less than the women who got a full seven hours of sleep.

What accounts for their weight gain? We don't know. It will take more studies to figure that out.