Kids who hit the hay early and wake with the roosters are less likely to become obese than those who go to bed late -- even if they still get the same amount of sleep, new research finds.

We've long heard that a good night's rest brings a wealth of health benefits, and a number of studies have shown that those who stay up into the wee hours tend to have more trouble with their weight.

But this study found it is not how long teens and children sleep that matters most, but what time they go to bed and then rise in the morning.

The research, which appears in the journal SLEEP, recorded the bedtimes and wake times of 2,200 Australian teens and "tweens" between the ages of 9 and 16. Researchers compared their use of free time over four days and measured their weight.

The early-to-bed, early-to-rise kids went to sleep about 70 to 90 minutes earlier than the late-risers, and woke up 60 to 80 minutes earlier.

The earlier risers also accumulated 27 minutes more moderate to vigorous physical activity a day. The late-to-bed and late-to-rise kids tended to watch TV, play video games or surf online 48 minutes longer each day than early-to-bed teens, primarily between 7 p.m. and midnight.

Late-nighters were almost twice as likely to be physically inactive and 2.9 times more likely to sit in front of the TV or computers for longer than guidelines recommend.

And all that inactivity seems to catch up. The kids who went to bed late were 1.5 times more likely to be obese than those who went to bed early and got up early, the researchers found.

Study co-author Carol Maher, a postdoctoral fellow at the University of South Australia, said the fact that the two sets of sleepers got roughly the same amount of shut-eye indicates that the timing of sleep "is even more important" when it comes to evaluating sleep and health outcomes.

According to Maher, morning hours are more conducive to getting physical activity because the most tempting television offerings are on in prime-time and late at night. Evening and late-night hours are also times that teens engage in online social networking.

"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 said in a statement. "Our findings show that this sleeping pattern is associated with unfavourable activity patterns and health outcomes, and that the adolescents who don't follow this sleep pattern do better."