Researchers tracked nearly 70,000 middle-aged women involved in the Nurses Health Study for 16 years, recording their weight every two years.

Those who slept for about five hours a night were 32 percent more likely to gain 33 lbs. in weight or more, and 15 percent more likely to become obese over the course of the study, which was by far the largest of its kind.

It had been suggested previously that people who sleep less compensate by eating more, but analysis of the data found that they ate less. The scientists also discounted physical activity as a reason to explain why women who slept less weighed more.

Less Sleep, Less Fidgeting?

They said further research into the cause of this apparent effect was needed, but suggested sleeping less may affect the body's metabolic rate or that it made people move around less during the day, burning fewer calories.

Professor Sanjay Patel, of Case Western Reserve University in Cleveland, who was the lead author of a report presented to the American Thoracic Society International Conference yesterday, said: "There have been a number of studies that have shown that at one point in time, people who sleep less weigh more, but this is one of the first to show reduced sleep increases the risk of gaining weight over time."

Professor Patel said the researchers had not established a causal link between getting less sleep and putting on weight.

"We don't have an answer from this study about why reduced sleep causes weight gain, but there are some possibilities that deserve further study," he said. "Sleeping less may effect changes in a person's basal metabolic rate -- the number of calories you burn when you rest.

"Another contributor to weight regulation that has recently been discovered is non-exercise associated thermogenesis, which refers to involuntary activity, such as fidgeting or standing instead of sitting," Prof Patel went on.

"It may be that if you sleep less, you move around less, too, and therefore burn up fewer calories."

Findings Viewed as 'Extraordinary'

Dr. Colin Waine, chairman of the National Obesity Forum, described the findings as "extraordinary." He continued: "It does surprise me. I would have thought those who sleep less would use more calories. It is an interesting study and one we have to look very hard at."

However, Dr. Waine said that those people who were looking to lose weight should address their diet and exercise habits first before considering the effect of sleep.

"I would say they'd want to think more about their activity patterns and their patterns of eating as well," Dr. Waine said.

"If you have a big main meal then go to sleep, you have no chance to use any of those calories."