Middle-aged women may be able to sleep their way to a trimmer body, new study findings suggest.

In a study that followed more than 68,000 U.S. women for 16 years, researchers found that those who caught more zzz's each night tended to put on less weight during middle-age.

What's more, women who typically clocked 5 hours of sleep were one third more likely than those who slept for 7 hours to have a substantial weight gain -- 33 pounds or more -- during the study period.

The findings, published in the American Journal of Epidemiology and presented earlier this year at a medical conference, add to evidence that sleep habits affect a person's weight.

Although the reasons aren't clear, some research suggests that sleep deprivation alters hormones involved in appetite control and metabolism.

It's also possible that people who sleep fewer hours either eat more or, because of fatigue, exercise less often.

Whatever the reason, the new findings suggest that sleeping 7 hours or more each night could help prevent the middle-age spread, according to the study authors.

Dr. Sanjay R. Patel of Case Western Reserve University in Cleveland led the research.

Patel and his colleagues based their findings on data from the long-running Nurses' Health Study, which has followed the health of thousands of female nurses for the past 30 years.

On average, women who in 1986 said they usually slept 5 hours or less per night gained more weight over the next 16 years than those who slept for 7 hours per night or longer.

Although the effect was modest, Patel's team notes, even a relatively small weight gain can make a health difference; putting on an extra 10 pounds has been shown to double a person's risk of diabetes, for example.

And some of the weight gain was substantial. Sleep-deprived women were more likely to gain in excess of 30 pounds, and were 15 percent more likely to become obese as they grew older.

Consuming extra calories could not be blamed for the weight gain, the investigators add, because women who slept less also ate less. Similarly, differences in levels of physical activity did not appear to be a factor.

"These findings," the researchers conclude, "have the important implication that increasing sleep time among those sleeping less than 7 hours per night may represent a novel approach to obesity prevention."

SOURCE: American Journal of Epidemiology, November 15, 2006.