But it was found people who slept four hours or less per night were 73% more likely to be obese, possibly because of effects on appetite hormones.

The Columbia University study was presented to the North American Association for the Study of Obesity annual meeting in Las Vegas.

Sleep deprivation

A team from the Mailman School of Public Health and the Obesity Research Center at Columbia analysed data on 18,000 people aged between 32 and 59 who participated in the National Health and Nutrition Examination Survey during the 1980s.

They found that, even after factors such as depression, physical activity, alcohol consumption, ethnicity, level of education, age and gender had been taken into account, people were more likely to be obese the less sleep they had.

While those who had less than four hours sleep were most at risk, people who got only five hours of sleep were 50% more likely to be obese than those who were getting a full night's rest.

Those who got six hours of sleep were 23% more likely to be substantially overweight.

Dr James Gangwisch, who led the research team, admitted: "The results are somewhat counterintuitive, since people who sleep less are naturally burning more calories."

"But we think it has more to do with what happens to your body when you deprive it of sleep as opposed to the amount of physical activity that you get."

And Dr Stephen Heymsfield, who also worked on the study, said it was not as simple as saying that if people were awake for longer, they were likely to eat more.

"There's growing scientific evidence that there's a link between sleep and the various neural pathways that regulate food intake."

Prehistoric cause?

He said previous research had shown sleep deprivation was linked to a decrease in levels of the hormone leptin, which regulates appetite and weight and tells the brain how much energy is available in the body.

However, levels of the hormone grehlin, which makes people want to eat, have also been seen to increase in people who are sleep-deprived.

Dr Gangwisch suggested the reason for this effect could hark back to prehistoric times.

"The metabolic regulatory system may have evolved to motivate humans to store fat during summer months when the nights are shorter and food is plentiful, which was a survival mechanism for the body to prepare for the dark winter months when food would not be as plentiful.

"As a result, sleeping less could serve as a trigger to the body to increase food intake and store fat."

Dr David Haslam, chairman of the National Obesity Forum, said: "This is counterintuitive. But I think the key to it is that, if you're not sleeping, you're quite likely to be snacking in front of the computer or the TV.

"Being stressed, which can affect sleep patterns, is also known to affect hormone levels."

But Dr Haslam said: "This is very interesting because it flies in the face of what you would expect."