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The benefits of gainful employment are many, but working hard may have a downside: an increased risk of heart attack.

A new study found that people who worked 11 hours or more a day were 67% more likely to have a heart attack or die of one over an average 12 years of follow-up, compared with people who worked standard seven- or eight-hour days.

The study followed more than 7,000 British civil service workers aged 39 to 62 who originally signed up between 1991 and 1993. At the start of the study, none of the participants had heart disease. Researchers screened the group for heart disease every five years until 2004, and also consulted hospital data and registries to track rates of heart attack and death. By the end of the study, 29 participants had died of heart disease and 163 had had heart attacks.

The 11-hour mark appeared to be key. Those who worked relatively long days — up to 10 hours a day — were not at significantly higher heart risk than those who worked less. But once workers crossed the 11-hour threshold, their heart disease risk went up.

What the researchers also wanted to know was whether adding information about people’s work hours would help improve the accuracy of standard heart disease risk calculations — which categorize people into low, moderate or high risk over 10 years — based on traditional risk factors like high cholesterol, high blood pressure and smoking. This risk calculation is known as the Framingham risk score.

Indeed, adding work hours to the calculation improved prediction by nearly 5%. The Boston Globe‘s Daily Dose blog reported:

Some 124 “low-risk” participants out of the 6,400 were re-classified as “moderate-risk” — defined as having a 5-year heart attack risk of 5 to 10 percent — when their long work hours were combined with their Framingham risk score. Interestingly, 85 of the nearly 400 moderate-risk folks who were also included in the study were re-classified as low-risk once their eight-hour-long workdays were factored in.

Why long work hours are associated with an increase in heart risk isn’t clear. It could be that working long days isn’t a risk factor in itself, but a marker for other things that increase risk: for instance, high-stress types who are already at increased risk for heart disease may simply work longer than other people. The study’s authors theorized that chronic stress may be a culprit, since stress affects metabolic function and can lead to depression and sleep problems — all of which are associated with increased risks of heart disease and heart attacks.

Of course, there’s also the issue of sedentary office work. In a study published in January in the European Heart Journal, researchers found that prolonged bouts of sitting — practiced by legions of office workers — led to increased risks of two major heart disease indicators: larger waist size and higher levels of blood fats. That was true even in people who exercised regularly outside of work hours.

What does that mean for you? Remember to get up and stretch or take a lap around the office once in a while, if you’re slogging long days at work. Even these small changes in behavior can lower your health risks, the authors of the January study found.

Also, try to punch out a little earlier than you otherwise would. That means putting down the BlackBerry at home and not checking email from the plane. It’s not worth the heartache.