A University of Otago researcher believes parents should keep a close eye on when their teenage boys hit the hay.
They've found teenage boys who sleep on average less than eight hours a night have nine percent more body fat than those who sleep more than eight hours.
Although, they've also found there's no discernible change in the body types of teenage girls who sleep less than eight hours a day.
Dr Paula Skidmore says parents should think about taking back the control of their teenage boys' bedtimes.
She says getting kids to turn off their TVs and close their eyes half an hour earlier could help a lot.