It looks like the old saying early to bed; early to rise actually has scientific merit.

A new study by researchers at the University of South Australia shows that children you went to bed and woke up early are more physically fit and active than those who stayed up later.

Previous studies have shown that children who got shorter sleep times are more prone to obesity and health problem but this is the first time that the time children sleep also impacts their health.

The study found that that children who got to bed early and woke up early were 1.5 times less likely to be obese than children who slept later and woke up later. Both groups had roughly the same sleep time- nine-and-a-half- hours.

It is still unclear why those early risers are more likely to be physically fit than their night-owl counterparts but researchers theorize that it has to do with the activities they are inclined to do. The researchers found that early to bed sleepers spent about 70 minutes a day being physically more active while late riser were more likely to spend more time in front of television screen and computers.

"We don't know if it's their sleep patterns have knock-on effects to the kinds of activities they do or if their activities affect their sleep patterns," said Carol Maher, the lead researcher based at the University of South Australia.

"All we can say is it's clear from our study the kids who have that early to bed, early to wake up sleep pattern are doing better and are less likely to be overweight or obese.

"It kind of goes against some wisdom we have heard that teenagers' sleep patterns are biologically different and it's normal for them not to go to bed early and be waking up late.

"There's been some schools experimenting with changing their starting times to fit in with teenagers' natural sleep patterns but this study suggests that is not perhaps a great idea."

The researchers divided the nine to 16 year olds participants into four groups: early to bed, early to wake; early to bed, late to wake; late to bed, early to wake; and late to bed and late to wake.

The average early bedtime was 9.20 p.m while early wakeups were 7a.m. Late bed times were around 10.40pm and late wake-ups at 8.20 a.m.