WEDNESDAY, June 25, 2014 (HealthDay News) -- Attention, binge TV watchers: New research suggests that long stretches spent glued to the tube may be more than just a guilty pleasure -- they could also shorten your life.

The study of more than 13,000 seemingly healthy adults in Spain found that those who spent more than three hours a day watching television had double the risk of early death compared to those who watched less than an hour a day.

"It is a little bit surprising," said study author Dr. Miguel Martinez-Gonzalez, a professor of preventive medicine at the University of Navarra, in Pamplona, Spain.

Gonzalez said the study participants were highly educated, and slim and active. Their average age was 37.

Anyone who said they had diabetes, heart disease or cancer was excluded at the start of the study.

Over the roughly eight years they were followed, there were 97 deaths -- 19 from heart disease, 46 from cancer, and 32 due to other causes such as accidents and liver or kidney disease.

Researchers then compared the risk of death with time spent in sedentary activities, such as watching TV, working at a computer and driving.

No link was found between early death and driving or working at a computer, according to the study, published online June 25 in the Journal of the American Heart Association.

But the risks associated with watching TV seemed striking. For every two additional hours a person spent watching TV, the risk of death from heart disease jumped by 44 percent, cancer death risk climbed by 21 percent and the risk of premature death went up by 55 percent for all other causes, compared to people who said they watched less than an hour each day.

All that just from watching TV? Gonzalez said even the researchers were skeptical about their findings.

They wondered, for example, whether people already gravely ill might watch more TV because they weren't physically able to do anything else. So they ran the numbers a second time, excluding the 35 deaths that happened in the first three years of the study, hoping to knock out people who might have been very sick. But the results only got stronger.