Women who sleep five hours or less each night are more likely to be become overweight or obese, a US study suggests.

Researchers from Ohio's Case Western Reserve University studied data on 68,183 middle-aged women who were enrolled in the Nurses Health Study.

They were asked in 1986 about their typical night's sleep, and were then asked to report their weight every 2 years for 16 years.

The team found women who slept for about five hours a night were 32 per cent more likely to gain 33lb in weight or more, and 15 per cent more likely to become obese over the course of the study compared with women who slept for seven hours.

Women who slept for six hours a night were 12 per cent more likely to have major weight gain and 6 per cent more likely to become obese compared with women who slept for seven hours a night.

Study findings were presented to the American Thoracic Society International Conference in San Diego on Tuesday (May 23).

On average, women who slept for five hours or less per night weighed 5.4 pounds more at the beginning of the study than those sleeping seven hours, and gained an additional 1.6 pounds more over the next 10 years.

"That may not sound like much, but it is an average amount - some women gained much more than that, and even a small difference in weight can increase a person's risk of health problems such as diabetes and hypertension," said lead researcher Dr Sanjay Patel.

Dr Patel noted that this is by far the largest study to track the effect of sleep habits on weight gain over time.

Dr Sanjay Patel, Case Western Reserve University: "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."

"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 studies to show reduced sleep increases the risk of gaining weight over time," Dr Patel said.

The researchers looked at the women's diets and exercise habits to see if they could account for part of the findings.

Dr Patel added: "Prior studies have shown that after just a few days of sleep restriction, the hormones that control appetite cause people to become hungrier, so we thought that women who slept less might eat more.

"But in fact they ate less. That suggests that appetite and diet are not accounting for the weight gain in women who sleep less."

The researchers also asked women about how much they participated in exercise activities such as running, jogging or playing tennis.

But they didn't find any differences in physical activity that could explain why women who slept less weighed more.

"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," Dr Patel said.

"Sleeping less may affect changes in a person's basal metabolic. Another contributor to weight regulation that has recently been discovered is called non-exercise associated thermogenesis, or NEAT, which refers to involuntary activity, such as fidgeting or standing instead of sitting.

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