Children who sleep for longer periods of time have a lower risk of obesity, a new study claims.
Researchers in the US found that every additional hour per night children aged between eight and nine spent sleeping cut their chances of being obese by the age of 11 or 12 by 40%.
Dr Julie Lumeng, from the University of Michigan, who led the study, said the less sleep the children got, the more likely they were to be obese in sixth grade, no matter what their weight was in the third grade.
She also found nine hours, 45 minutes of sleep was the "magic number", as sleeping more than that lowered the risk of obesity significantly.
The findings, published in the journal Paediatrics, will encourage parents to restrict caffeine intake, and enforce regular bedtime hours.
Other studies have shown that a lack of sleep can affect two hormones that regulate appetite, and this may be one factor behind the results. Another explanation is that tired children are less likely to exercise.
Dr Stephen Sheldon, director of sleep medicine at Chicago's Children's Memorial Hospital, said children's sleep may be disturbed by breathing problems - some caused by being overweight, such as sleep apnoea, and some caused by enlarged tonsils and adenoids.
"I'm not so sure we have enough information yet on cause and effect," he added.