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Sitting in front of the television may be a relaxing way to pass an evening, but spending too much time in front of the tube may take years off your life.

That’s what Australian researchers found when they generated life-expectancy tables for people based on mortality information from the Australian Bureau of Statistics as well as participants’ survey responses about how much TV they had watched in the past week.

The TV-viewing data from more than 11,000 participants older than 25 years showed that Australian adults watched an estimated 9.8 billion hours of television in 2008. People who watched an average six hours of TV a day lived an average 4.8 years fewer than those who didn’t watch any television, the study found.

Even more humbling: every hour of TV that participants watched after age 25 was associated with a 22-minute reduction in their life expectancy.

(MORE: Too Much TV Linked with Disease and Early Death)

The findings suggest that watching too much TV is as detrimental to longevity as smoking and lack of exercise. Previous research has shown that smoking is associated with a four-year reduction in life expectancy after the age of 50. That works out to an average 11 minutes of life lost for every cigarette smoked — the equivalent to 30 minutes of TV time, according to the current study.

The study notes also that people who report low levels of physical activity lose nearly 1.5 years in life expectancy compared with those who exercise a moderate amount, an effect similar to that of watching just over two hours of television a day.

“The strong correlation is a bit of a surprise,” said lead author Lennert Veerman of the University of Queensland in an e-mail response to questions about his research, which was published in the British Journal of Sports Medicine. “It suggests that going from inactive to slightly active is as important as exercise.”

It’s no mystery that sitting in front of the tube isn’t exactly a healthy pursuit. The more TV you watch, the less physically active you are. And the less exercise you get, the more likely you are to develop diseases such as diabetes or heart problems.

But while previous studies have hinted at the potentially deadly impact of too much TV watching — in June, a Harvard study found that for every two hours of TV watched, people’s risk of dying from any cause increased 13% over a seven-year period — the new analysis was the first to translate the effect of TV viewing to life expectancy at birth.

Were it not for TV, researchers estimated that life expectancy for men would be 1.8 years longer and for women, 1.5 years longer.

(MORE: Even a Little Bit of Exercise Goes a Long Way)

Veerman acknowledges that it may not just be the sedentary nature of watching TV that lowers life expectancy but also the poor diet that onscreen junk-food advertising can promote. Still, the association between excess TV viewing and lower life expectancy persisted, even after adjusting for diet, Veerman says.

He says it might make sense for doctors to start asking their patients about how much time they spend in front of the TV and to treat TV time as they would other risk factors for poor health, such as lack of exercise and an unhealthy diet.

Also, Veerman notes, the dangers of TV viewing can easily be neutralized by simply turning off the TV and getting off the couch. “Exercise is good,” he says, “but even light physical activity also improves health.”

MORE: Just 15 Minutes of Exercise a Day May Add Years to Your Life

Alice Park is a writer at TIME. Find her on Twitter at @aliceparkny. You can also continue the discussion on TIME‘s Facebook page and on Twitter at @TIME.