Children and teens who go to bed late and wake up late are more likely to be overweight than their peers who go to bed early and rise early, Australian researchers suggest.

For the study, 2,200 Australians aged nine to 16 kept track of their bedtimes, wake times as well as time spent watching TV, playing videogames or using computers. They also wore pedometers to record their physical activity levels, and weights were measured.

When they went to sleep was key to the results.

Those who went to bed late and got up late were 1.5 times more likely to become obese than those who went to bed early and got up early, even when they got the same amount of sleep, Carol Maher, a post-doctoral fellow with the University of South Australia, and her co-authors reported in Saturday's issue of the journal Sleep.

The late-nighters were almost twice as likely to be physically inactive and nearly three times more likely to sit in front of screens for more than guidelines recommend, racking up nearly an hour more of these sedentary activities.

"This study shows that the teenagers that have that pattern of late to bed, late to wake up don't have as good health outcomes," Maher told the Australian Broadcasting Corporation.

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The time teens go to bed is linked to their risk of being overweight. ((Jessica Rinaldi/Reuters))

The findings contradict opinions that it is normal for teenagers to get into a sleeping pattern of staying up late and sleeping in, or advisable to adjust school times to fit in with a teenager's sleep patterns, she said.

Since the study was observational in design, it is difficult to determine what could be causing the effect, said CBC's medical specialist Dr. Karl Kabasele.

"What's not clear is perhaps maybe it's the kids who like to do exercise who just choose to get up early and go to bed early," Kabasele said. "Kids who stay up late can't really exercise."

Screen time, diet, parenting factors

The study's authors said that young people who are attracted to physical activity may choose to get up early to take part in early morning training sessions or to walk or bike to school. Similarly, young people who prefer screen time may choose to stay up late to watch TV or log on to social networking.

External factors in the home may also make a difference, such as parents' need to leave early for work, siblings' activities and household rules around bedtime, the researchers said.

The findings point to the importance of having parents encourage healthy habits in their children, Kabasele said.

Dr. Louise Hardy, a senior research fellow at the Prevention Research Collaboration based at the University of Sydney, studied childhood obesity rates in New South Wales.

There is a strong association between sedentary activities such as television viewing and obesity, Hardy said.

The researchers "haven't got dietary data, but it would be interesting to see if those children going to bed late are actually in front of a television or maybe they're social networking where they can get their hands off the computer to eat."

The investigators called for long-term studies to confirm how bed and wake time habits may affect weight, as well as experiments that manipulate bedtimes to shed light on the possible cause-and-effect mechanisms.

The study was funded by the Australian Commonwealth Department of Health and Aging; the Department of Agriculture, Fisheries and Forestry; and by the Australian Food and Grocery Council, an industry sponsor that the researchers said played no role in preparing the manuscript.

With files from the Australian Broadcasting Corporation