Watching too much television increases your risk of dying from a heart attack or stroke, researchers have found.
And sitting down to watch the small screen is worse than doing so at work or when driving because viewers tend to snack at the same time, the data suggests.
Couch potatoes may also do less exercise, while blue light from a television screen can disrupt their sleep.
The research looked at 490,966 people, aged 37 to 73, who signed up for the UK Biobank study.
They reported how much television they watched in a typical day and the data was then analysed.
Those who watched more television had greater odds of developing cardiovascular disease, such as heart attacks or strokes, and dying from them.
They were also more likely to develop cancer or die from it – principally lung cancer.
Researchers concluded that if everyone in the study had watched no more than two hours of television a day, almost eight per cent of deaths from cardiovascular disease could have been prevented.
And more than five per cent of the deaths overall would not have taken place.
Researchers discovered that if someone who walked for only half an hour a day swapped half an hour of television time for a walk, they could cut their risk of dying by 10 per cent.
It also appeared that for those who slept less than six hours a night, it was healthier to swap TV viewing for some shut-eye.
Dr Hamish Foster, who led the study from the University of Glasgow’s Institute of Health and Wellbeing, said: ‘Our study has found that time spent watching television is associated with a number of bad health outcomes.
‘[The research] supports the idea that people who watch more than two hours of television could replace some of that time with more walking or sleep, if they don’t do a lot of either, to potentially improve their health.’
But Dr Joy Leahy, of the Royal Statistical Society, said: ‘It is unlikely that one study alone will be strong enough evidence to state definitively that watching less than exactly two hours of TV a day will minimise health risks.’
The research was published in journal Mayo Clinic Proceedings.