Soaring levels of child obesity may be caused by youngsters getting too little sleep, claim researchers.
Children today get fewer hours of sleep than in the past and less than they need, which can disrupt hormones that help control weight gain, said Dr Shahrad Taheri from Bristol University.
He blames TV viewing, computer games and mobile phones for keeping children up later and affecting the quality of their sleep.
But there is growing concern about a 'couch potato' generation leading sedentary lifestyles dominated by the 'electronic babysitter' of TV and computer screens.
Dr Taheri, of the university's Henry Wellcome Institute, said parents need to impose stricter controls on their children's sleeping routines, which could include removing electronic gadgets from the bedroom.
He said "Sleep is probably not the only answer to the obesity pandemic, but its effect should be taken seriously, as even small changes in energy balance are beneficial."
One in four children aged 11-15 years is now obese - so fat it threatens their health - with almost half of girls officially classified as obese or overweight.
Writing in the Archives of Disease in Childhood, Dr Taheri said mounting evidence suggested lack of sleep was a plausible contributory factor to the rise in obesity among children, with effects that led to weight gain in later life.
Studies show sleep loss could disturb the production of hormones that control the desire for calorie-rich foods, hunger and energy expenditure.
For example, the level of ghrelin, a hormone released by the stomach to signal hunger, was found to be 15 per cent higher in people who have only five hours sleep a night than those getting eight hours.
Lack of sleep also leads to tiredness during the day, which may mean people may not have sufficient energy to do physical activity, a major contributor to obesity.
Dr Taheri said there was an emerging body of evidence that looked at the 'real life' effect of a fall in the nightly quote of sleep - rather than the extreme effects of sustained sleep deprivation.
The link between obesity and too little sleep appears to be particularly strong in children and young people, he added.
The Avon Longitudinal Study of Parents and Children in the UK found in the 1990s that insufficient sleep at 30 months predicted obesity at age seven, showing it could programme the part of the brain regulating appetite and energy expenditure.
Teenagers are also at risk because they need more sleep at a critical period of development.
Evidence shows that as little as two or three nights of sleep restriction could have a 'profound effect' on young adults, Dr Taheri said.
Good sleep could be promoted by removing televisions and other electronic items from children's bedrooms and ensuring a strict, regular bedtime routine, he said.
He said "Ensuring adequate sleep in children and adolescents may not only help fighting against obesity, but could have other added health and educational benefits - for example improvements in academic performance.
"An obesity prevention approach in children and adolescents that promotes a healthy diet, physical activity and adequate sleep could be adopted."