Four hours' screen time a day more than doubles the risk of a heart attack, according to scientists at University College London.
The team says the danger appears to be independent of how much exercise a person gets when away from the screen, but suggests that metabolic factors and inflammation could cause a link between prolonged sitting and the risks to heart health.
"People who spend excessive amounts of time in front of a screen - primarily watching TV - are more likely to die of any cause and suffer heart-related problems," says Emmanuel Stamatakis of UCL's Department of Epidemiology and Public Health. "Our analysis suggests that two or more hours of screen time each day may place someone at greater risk for a cardiac event."
In fact, compared with those spending less than two hours a day on screen-based entertainment, there was a 48 percent increased risk of all-cause mortality in those spending four or more hours a day and an approximately 125 percent increase in risk of cardiovascular events in those spending two or more hours a day.
The increase appeared independently of traditional risk factors such as smoking, hypertension, BMI, social class, as well as exercise.
The data indicate that a quarter of the association between screen time and cardiovascular events was explained collectively by C-reactive protein (CRP), body mass index, and high-density lipoprotein cholesterol - suggesting that inflammation and deregulation could be one cause.
CRP, a well-established marker of low-grade inflammation, was twice as high in people spending more than four hours of screen time per day as in those spending less than two hours a day.
Dr Stamatakis says the next step will be to try to uncover what prolonged sitting does to the human body in the short- and long-term, whether and how exercise can mitigate these consequences, and how to alter lifestyles to reduce sitting and increase movement and exercise.
"It is all a matter of habit. Many of us have learned to go back home, turn the TV set on and sit down for several hours - it's convenient and easy to do. But doing so is bad for the heart and our health in general," said Dr Stamatakis.
"And according to what we know so far, these health risks may not be mitigated by exercise, a finding that underscores the urgent need for public health recommendations to include guidelines for limiting recreational sitting and other sedentary behaviors, in addition to improving physical activity."