Thousands of Scottish secondary pupils spend more time during the school day staring at a TV, computer, console or mobile phone than they do in class.
A worrying new survey of S1-4 pupils in Glasgow has found that almost half of those whose responses were validated said that screen-based activities occupied at least eight hours of an average school day.
Of those, hundreds - or one in four - confessed that they looked at a screen for 12 hours or more.
The most common screen-related activity was watching TV, followed by surfing the net and social networking, although homework accounted for more screen time than computer games.
Education and health chiefs admitted that such a "massive" amount of time on mostly sedentary activities was "worrying".
However, they stressed that the same survey found an "encouraging" rise in the number of pupils participating in physical activity.
There was also a "significant" drop in the number of pupils who reported taking drugs or alcohol or smoking.
That said, serious concerns were expressed after it emerged that three-quarters of youngsters regularly suffer the effects of second-hand smoke at home - driving a rise in asthma among schoolchildren.
Fiona Moss, head of health improvement at NHS Greater Glasgow and Clyde, which conducted the survey in partnership with Glasgow City Council, said: "It is a massive amount of screen time. Families may have a limit for internet or television time but they don't tend to think of screen time as a totality.
"I would say that overall this is a really positive report. Some of the improvements (in relation to drinking, drugs, smoking and sport) are only 3 or 4 per cent better but we're talking about a big population. That is significant."
Ms Moss added: "Bike ownership and physical activity reduce as you go further up the school. We have to watch that screen time does not replace these activities."
The health and well-being survey of 8,282 pupils across 30 Glasgow secondaries was a follow-up to an initial survey conducted in 2007.
New questions on screen time revealed that of 4,638 youngsters who gave valid answers, 49 per cent (2,273) spent at least eight hours looking at a TV, computer, console or mobile, while 25 per cent (more than 1,000) spent 12 or more hours in front of a screen.
Maureen McKenna, head of education at Glasgow City Council, said work to make pupils more active should help reduce the huge amount of screen time.
"Universally, children are spending too much time using a computer, so anything we can do to encourage them to be more active is going to be beneficial," she said.
"We have a number of cycling initiatives driven by the Commonwealth Games and I think the finding that 73 per cent of pupils now own a bike - up from 71 per cent in 2007 - is very positive."
Efforts to reduce teenage drinking were also working, she said, with the proportion of pupils reporting that they never drank alcohol rising from 46.5 per cent to 61 per cent.
But she warned that the impact of secondary smoking was a "more worrying" issue than screen time.
The finding that 74 per cent of pupils were exposed to second-hand smoke was linked to a rise in the number of youngsters reporting a long-term illness, up from 22 per cent to 26 per cent.
Asthma was the most common condition, prompting a warning to parents about the damaging effects of their behaviour on their children.
- 65 per cent take part in a sports club run by school.
- 48 per cent of all young people travel to school by active means.
- 74 per cent eat a breakfast each morning.
- 83 per cent have received relationships education.
- 75 per cent have never tried smoking.
- 61 per cent have never drunk alcohol.
- 74 per cent are exposed to second-hand smoke indoors often or every day.
- 25 per cent spend more than 12 hours on screen-based activities.