Study co-author Carol Maher, a postdoctoral fellow at the University of South Australia, and colleagues recorded the bedtimes and wake-up times of 2,200 Australian study participants ages 9-16. They compared their weight and use of free time over four days.
The study, published in the journal Sleep, found children 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.
Late-nighters were almost twice as likely to be physically inactive and 2.9 times more likely to sit in front of the TV and computer or play video games for more hours than guidelines recommend, the study said.
"The children who went to bed late and woke up late, and the children who went to bed early and woke up early got virtually the same amount of sleep in total," Maher said in a statement. "Scientists have realized in recent years that children who get less sleep tend to do worse on a variety of health outcomes, including the risk of being overweight and obese. Our study suggests that the timing of sleep is even more important."