As they turn to handheld digital devices for their entertainment, they are just as much at risk of being fat and having an unhealthy lifestyle as if they were plonked in front of the gogglebox.
Those aged 14 to 18 who spent more than five hours a day on screen devices were twice as likely to drink a sugary drink each day and not get enough sleep or physical exercise.
They were more than two fifths - 43 per cent - more likely to be obese than their peers who did not spend time on these devices.
Research fellow Dr Erica Kenney and Professor Steven Gortmaker at Harvard University said time spent on the devices “saturates the waking hours of most American youth.”
Yet many spend over six hours on them - well over the two-hour limit recommended by the American Academy of Pediatrics.
Dr Kenney said: “The types of screens that youthS use have changed rapidly.
“Smartphone use now makes up the majority of screen engagement among teens, and most youth, even younger children, have their own device; a recent study found over one-half of three year olds had been given their own tablet.”
She added excessive TV watching was linked with heightened risk of childhood obesity, a higher intake of total energy, as well as sugar sweetened beverages (SSBs), fast food, sweets, and salty snacks.
It was also linked with “shorter and poorer quality sleep,which is itself associated with excess energy intake and weight gain.”
She said: “Less is known about the health impacts of other screen devices, including computers, videogames, and newer media devices such as smartphones and tablets.
“Existing evidence suggests associations between smartphone and tablet use and inadequate sleep, and food and beverage advertising is highly prevalent among social media and other websites directed toward children and youth.
“Given the pervasiveness of TV and other screen devices in the lives of American youth, and given that the majority of adolescents’ total screen time is now spent engaging with smartphones - about 2.75 hours per day, on average - it is important to understand how evolving screen devices impact health.”
The study followed 24,800 teenagers in the 2013 and 2015 waves of the Youth Risk Behaviour Surveillance System.
Data included hours spent on screen devices and watching television, hours of sleep on an average school night, number of SSBs consumed in the previous seven days, and frequency of physical activity (at least 60 minutes per day) for the past seven days.
It found a fifth of teenagers spent more than five hours a day on the devices with less than a tenth - eight per cent - watching more than five hours of television.
Over a quarter of boys and nearly a fifth of girls drank at least one SSB a day and approximately two-thirds of boys and three quarters of girls did not get any daily physical activity.
More than two-thirds got less than eight hours of sleep each night and overall an eighth - 13.6 per cent - were obese.
Dr Kenney said: “TV viewing was associated with daily intake of SSBs and obesity among both boys and girls, and with physical inactivity among girls, but, interestingly, TV viewing was actually associated with lower risk of physical inactivity among boys, and possibly lower risk of inadequate sleep.
“In contrast, higher use of other screen devices, including computers, videogames, smartphones, and tablets, was significantly associated with all obesity risk factors studied for both boys and girls, including SSB intake, sleep, and physical inactivity.
“The use of other screen devices is linked with several aspects of adolescent health and well-being, and TV time is more narrowly linked with diet and obesity.
“This study would suggest that limiting children’s and adolescents’ engagement with other screen devices may be as important for health as limiting television time.”
The study was published in The Journal of Paediatrics.