Early to bed and early to rise, makes a teen healthy and ... less prone to obesity.

Indeed, teenagers who go to bed later, even if they get the same amount of sleep, tend to be less physically fit and fatter.

Researchers from the University of South Australia gathered data on 2,000 children from ages 9 to 16, as reported in the journal Sleep. Those who kept later hours were 2.9 times more likely to be physically inactive and 1.5 times more likely to become obese.

The study noted that night owls spend about 48 minutes longer watching TV, going online or playing video games than their early-bird peers. They essentially replaced 30 minutes of moderate to vigorous physical activity with sedentary pursuits each day.

Those who regularly kept earlier hours did almost 30 minutes more exercise each day.

Research demonstrating the health benefits of sleep is not new. Shakespeare knew that sleep knits "the raveled sleeve of care." For teens, the magic number is somewhere between nine and 10 hours of sleep a night.

But this new research indicates it isn't just how much sleep a teen gets but when he or she gets it that makes a difference.

So, teenagers, save up your allowance for TiVo so you can record your favorite late-night shows and hit the hay by about 9 p.m.

You'll like what you see in the mirror.