Teenagers who get fewer than six hours sleep a night are 20 per cent more likely to be obese later in life.

Researchers found that by age 21, people who got eight hours a night were significantly less likely to be carrying extra pounds. The Centers for Disease Control and Prevention in the U.S. recommends teenagers get nine to ten hours of sleep each night.

In the new study, researchers at the Columbia University and the University of North Carolina analysed health information from more than 10,000 Americans aged 16 and again at 21.

Information on height and weight and sleep was collected during home visits in 1995 and 2001. Nearly one-fifth of the 16-year-olds reported getting less than six hours of sleep. And this group was 20 per cent more likely to be obese by age 21, compared to their peers who got more than eight hours of sleep.

While lack of physical activity and time spent watching television contributed to obesity, they did not account for the relationship between sleeplessness and obesity. Daytime sleepiness and fatigue are known to affect what and how people eat, by altering appetite and stimulating cravings. Energy levels may also play a role. For the sleep-deprived, ordering calorie-dense fast food is easier than preparing a nutritious meal.

'Lack of sleep in your teenage years can stack the deck against you for obesity later in life,' said Shakira Suglia, assistant professor of epidemiology at Columbia University. 'Once you’re an obese adult, it is much harder to lose weight and keep it off. And the longer you are obese, the greater your risk for health problems like heart disease, diabetes, and cancer.'

'The message for parents is to make sure their teenagers get more than eight hours a night,' adds Dr Suglia.  'A good night’s sleep does more than help them stay alert in school. It helps them grow into healthy adults.'

Information on what the teenagxers ate was not recorded in the surveys, although it could play a role. The researchers now plan to examine whether fizzy drink consumption disrupts sleep - and in turn has an effect on obesity. The study is published in the Journal of Pediatrics.