Increases in total screen time between survey periods were linked with similar but smaller changes in BMI.

"The weight of the evidence is pretty strong that television viewing is related to unhealthy changes in weight among youth," says Jennifer Falbe, who led the study while at the Harvard School of Public Health in Boston. She is now at the University of California, Berkeley School of Public Health.

"It's important for parents to be aware of all the potentially obesogenic screens that they should really be limiting in their children's lives," Falbe says.

When children watch TV, "there is more purposeful, deliberate exposure to eating options, commercials that come on that might cue you to go off to the pantry and grab a cookie or a soft drink," Tremblay says.

"Typically your hands are free when you're watching TV, so should that temptation capture you, you're able to sit there and munch on whether it's a healthy or an unhealthy snack."

What's more, he says, "You can get into a pretty much hibernative state on the couch."

Even if children are sitting down while playing a computer game, for instance, they might be a bit more active, Tremblay says.