Researchers found that every additional hour a nine-year-old spends sleeping per night cuts by nearly 40 per cent the child's chances of being obese as a 12-year-old.

'Many children aren't getting enough sleep, and that lack of sleep may not only be making them moody or preventing them from being alert and ready to learn at school, it may also be leading to a higher risk of being overweight,' said Dr Julie Lumeng of the University of Michigan.

She led the study, whose results appeared in the journal Pediatrics.

The findings were based on an analysis of sleep patterns and other data from a government survey of 785 children aged nine to 12.

The 12-year-olds who slept less than nine hours a night were more likely to be overweight than those who slept more, the study found.

Those who got less sleep at age nine were more likely to be overweight three years later.

The impact of sleeping less was consistent regardless of race, gender, socio-economic status or home environment quality. The study recommended at least nine hours and 45 minutes of sleep per night.

Endocrinologist Eve Van Cauter of the University of Chicago, who was not involved in the study, said lack of sleep plays havoc with two hormones that regulate appetite.

Experiments by her and others found that sleep-deprived adults produce more ghrelin, a hormone that promotes hunger, and less leptin, a hormone that signals fullness.

Dr Lumeng said tired kids are less likely to exercise and more likely to sit on the couch and eat cookies.

REUTERS, ASSOCIATED PRESS