'Sleep more and stay slim'

Getting a good night's sleep may be one of the simplest ways to stay slim, new research suggests.

A study found that women who slept too little - five or fewer hours per night - are at risk of major weight gain. Light sleepers weighed more on average than those who slept for seven hours.

Almost 70,000 women took part in the research, part of a major health investigation in the US called the Nurses Health Study.

Scientists monitored the women for 16 years, keeping records of their weight and sleep patterns.

Compared with the sound sleepers, women who slept no more than five hours a night were 32 per cent more likely to experience major weight gain - defined as an increase of 33 pounds or more - during the course of the study.

They were also 15 per cent more likely to become obese compared with women who slept seven hours.

Obesity is defined as having a Body Mass Index (BMI), a measurement relating weight and height, of 30 or more.

The findings had nothing to do with light sleepers eating too much, or taking too little exercise, the researchers found.

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.

Dr Sanjay Patel, from Case Western Reserve University in Cleveland, Ohio, who led the study, 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)."

The study is by far the largest to track the effects of sleep habits on weight gain over a long period of time.

Dr Patel presented his findings today at the American Thoracic Society International Conference in San Diego, California.

He said his team was surprised to find no connection between the trend and diet.

"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," he said.

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

Exercise

Questioning the women about how much exercise they took revealed that physical activity also played no part in the findings.

Dr Patel added: "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.

"Sleeping less may affect 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 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."