FRIDAY, Sept. 30 (HealthDay News) -- Going to bed early and waking up early may help teens stay thinner and more physically active than their night-owl peers, and this was the case even when both groups got the same amount of sleep, researchers say.

"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," study co-author Carol Maher, a postdoctoral fellow with the University of South Australia, said in a news release from the American Academy of Sleep Medicine.

"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," Maher added.

In conducting the study, researchers collected information from 2,200 Australian children and teens aged 9 to 16 about when they go to bed and when they wake up. They also compared the kids' weights and the activities they used to fill their free time over the course of four days.

The study, published in the Oct. 1 issue of the journal Sleep, found that the night owls who also slept in later in the morning were 1.5 times more likely to become obese than the kids who were early to bed and early to rise.

In addition to gaining weight, teens who burned the midnight oil were also twice as likely to be sedentary and nearly three times as likely to spend an excessive amount of time watching TV, using the computer or playing video games.

The researchers suggested that early birds may tend to be leaner and more active because they are not distracted by prime-time TV shows and evening social networking.

The study authors warned that teens have a natural inclination to stay up late and wake up late, which could be worrisome since the activities available at those times are primarily sedentary and screen-based.

"It is widely accepted that the sleep patterns of adolescents are fundamentally different from children and adults, and that it is normal for adolescents to stay up very late and sleep in late in the morning," Maher explained in the news release. "Our findings show that this sleeping pattern is associated with unfavorable activity patterns and health outcomes, and that the adolescents who don't follow this sleep pattern do better."

Among the study's other key findings:

• Early birds went to bed up to 90 minutes earlier, woke up at least an hour sooner and got 27 minutes more vigorous exercise each day than their night-owl peers.
• Night owls who slept in logged 48 more minutes of screen time (TV, computer or video games) between 7 p.m. and midnight than kids who go to bed earlier.
• Only 12 percent of night owls got the expert recommended average of two hours or less of screen time daily, compared with 28 percent of early birds.
• Night owls generally substituted 30 minutes of moderate-to-vigorous exercise with 30 minutes of physical inactivity each day.
• Kids who go to bed late and sleep in had higher body mass index scores (a measurement that takes into account height and weight) than early risers and were more likely to be overweight or obese.
• Night owls typically had fewer sisters or brothers, lived in major cities, came from lower-income families and held a part-time job.

More information

The National Sleep Foundation provides more information on teens and sleep.