The study’s findings shocked its authors, who had concluded in 2005 that use could not possibly grow further, and confirmed the fears of many parents whose children are constantly tethered to media devices. It found, moreover, that heavy media use is associated with several negatives, including behavior problems and lower grades.

The third in a series, the study found that young people’s media consumption grew far more in the last five years than from 1999 to 2004, as sophisticated mobile technology like iPods and smart phones brought media access into teenagers’ pockets and beds.

Dr. Michael Rich, a pediatrician at Children’s Hospital Boston who directs the Center on Media and Child Health, said that with media use so ubiquitous, it was time to stop arguing over whether it was good or bad and accept it as part of children’s environment, “like the air they breathe, the water they drink and the food they eat.”

Contrary to popular wisdom, the heaviest media users reported spending a similar amount of time exercising as the light media users. Nonetheless, other studies have established a link between screen time and obesity.

While most of the young people in the study got good grades, 47 percent of the heaviest media users  those who consumed at least 16 hours a day  had mostly C’s or lower, compared with 23 percent of those who typically consumed media three hours a day or less. The heaviest media users were also more likely than the lightest users to report that they were bored or sad, or that they got into trouble, did not get along well with their parents and were not happy at school.