Not sleeping appears to be bad news.

A good night's sleep may not just leave you feeling refreshed - it may also help to you keep trim. Researchers from Ohio's Case Western Reserve University, followed nearly 70,000 women for 16 years. They found women who slept five or fewer hours a night were a third more likely to put on at least 33lbs (15kg) than sound sleepers during that time. Details were presented to the American Thoracic Society International Conference in San Diego. The study is by far the largest to track the effects of sleep habits on weight gain over a long period of time. It also found that lighter sleepers were 15% more likely to become obese compared with women who slept for seven hours a night. Obesity is defined as having a Body Mass Index (BMI) of 30 or more. BMI is calculated by dividing weight in kilograms by the square of height in metres. The researchers found that their findings had nothing to do with light sleepers eating too much, or taking too little exercise. On average, women who slept 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. Lead researcher Dr Sanjay Patel said: "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 (blood pressure)." "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." Dr Patel said sleeping less might alter the number of calories a person burns when at rest. Alternatively, he said people who sleep less might also be less prone to involuntary activity such as fidgeting, which can help to burn off calories. A study by the US National Institute of Mental Health, published in the journal Sleep in 2004, produced similar findings. The researchers suggested the key might be that sleep deprivation alters the balance of hormones which control the rate at which we burn off calories. Dr Ian Campbell, a GP in Nottingham and medical director of the charity Weight Concern, said it might be that people who sleep less are more tired, and therefore less active during the day. There may also be a natural hormonal response to the stress of insufficient sleep leading to changes in metabolic rate or even eating behaviors. "What is clear though is that so much of the way we live our lives affects our health," he said. "If we want to keep the risk of weight gain at bay there's nothing to beat a healthy diet, an active lifestyle, and a good night's rest." Dr Andrew Cummin, of the Sleep Laboratory at Charing Cross Hospital, said: "Many women would like to know the secret of eating more without gaining weight. "Sadly, the authors have no explanation. But it does seem that if you want to lose weight getting plenty of sleep may help."