You may think binge-watching the latest Netflix series is a guilty but harmless pleasure.

Think again, you popculture coach potatoes.

It could kill you. Really.

Oh, one or two episodes of House of Cards—or BoJack Horseman—won’t do you in. But too many hours of binge watching can greatly increase your risk for an often-fatal condition.

THE FIVE-HOUR THRESHOLD

Researchers at Osaka University in Japan were studying lifestyle factors related to blood clots. When a blood clot happens in the leg, it’s called deep vein thrombosisbut if one breaks free and travels to your lungs, it can cause a pulmonary embolism, which often is fatal.

Researchers already knew that being sedentary for a long period of time—such as a long airplane flight—can increase the odds of getting blood clots. So they decided to look at television watching. They analyzed records of more than 86,000 men and women between the ages of 40 and 79 in 45 regions of Japan.

Bingo: Those who watched between two-and-a-half and five hours of TV a night, on average, were 70% more likely than those who watched less than that to have a fatal pulmonary embolism…and those who watched more than five hours were 250% more likely than those who watched less than two-and-a-half hours of TV.

ENJOYING TV…IN MODERATION

Granted, the absolute number of deaths was still pretty small. There were 59 total deaths from pulmonary embolism over the 20 years of the study, 40 of them from the 2.5-hours-to-five hours and more-than-five-hours groups combined. Plus, these kinds of observational studies can’t actually prove cause and effect.

On the other hand, we already know that watching too much TV is remarkably bad for health—people who watch more than six hours a night on average are likely to die nearly five years earlier than TV abstainers, for example.

We’ve got nothing against relaxing in front of the TV. But if you find yourself zoning out for more than two or three hours every night, finding more active pursuits can be a very healthy change—it might even make you happier. At the very least, get up and walk around frequently—and try a few quick exercises.