Researchers found that among adults younger than 40, those who typically slept for five hours or less each night had a greater accumulation of belly fat over the next five years.

But those who logged eight hours or more in bed each night also showed a bigger fat gain, although it was less substantial than that seen in "short sleepers."

The study does not prove that too little or too much sleep directly leads to excess fat gain. But the findings support and extend those of other studies linking sleep duration -- particularly a lack of sleep -- to weight gain and even to higher risks of diabetes and heart disease.