Kids who follow Ben Franklin’s “early to bed, early to rise” principle are ultimately leaner and in better shape than those who stay up late and sleep in, according to a new study by Australian researchers.

Even though both would be getting the same amount of sleep, preteens and teens who go to bed earlier and wake up earlier are slimmer and more physically active than their peers who do not, according to the study published in the Oct. 1 issue of the journal SLEEP .

The study does not necessarily reveal that later bedtimes cause obesity, or that a child can slim down by hitting the sack earlier. The kids who go to bed early might enjoy rigorous physical activity, and adjust their sleep schedule accordingly, while late-nighters could be staying up for activity such as  reading or a television show they enjoy.

The study observed 2,200 Australian participants, ages 9 to 16, during four days, looking at their weights and uses of free time. Researchers found that the “night owls” were 1.5 times more likely to be obese than the “early birds.”

Late-nighters were approximately twice as likely to be physically inactive and almost three times as likely to participate in inert activities — such as surfing the web or playing video games — than recommended.

The purpose of the study was less about the amount of sleep in relation to activity and health, but timing, according to Carol Maher, a co-author of the study and postdoctoral fellow with the University of South Australia.

“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.

“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,” she added.

The amount of available physical activities that take place during daytime hours could be a factor, according to the researchers, while sports and exercising are more likely to take place during the day, sedentary, screen-based behaviors are more associated with nighttime.

The study, researchers say, can be taken as a warning, as research shows that teens naturally tend to stay up late and wake later.