Those ages 8 to 18 spend more than seven and a half hours a day with such devices, compared with less than six and a half hours five years ago, when the study was last conducted. And that does not count the hour and a half that youths spend texting, or the half-hour they talk on their mobile phones.

”I feel like my days would be boring without it,” said Francisco Sepulveda, a 14-year-old Bronx eighth grader who uses his smart phone to surf the Web, watch videos, listen to music — and send or receive about 500 texts a day. Yes you read that 500 tests per day…my fingers are tired just thinking about that!

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.

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.

On average, young people spend about two hours a day consuming media on a mobile device, the study found. They spend almost another hour on ”old” content like television or music delivered through newer pathways like the Web site Hulu or iTunes. Youths now spend more time listening to or watching media on their mobile phones, or playing games, than talking on them.

The Kaiser study found that more than 7 in 10 youths have a TV in their bedroom, and about a third have a computer with Internet access in their bedroom.