Working overtime doesn’t just stress you out—it also stresses your heart, according to study findings published online in the European Heart Journal and reported in press information from the European Society of Cardiology.

In the ongoing 25-year study, researchers followed more than 10,000 British civil servants to see how their work and social habits affected their health.

Researchers found that people who worked three or more hours of overtime were 60 percent more likely to suffer from heart attacks or angina (chest pain) than those who didn’t do overtime. (This wasn’t connected to other risk factors for heart trouble, such as smoking, having high cholesterol or being overweight.)

Why does overtime lead to heart trouble? Researchers aren’t sure. But they found that people who worked overtime often had tense, competitive type A personalities, showed signs of anxiety or depression or didn’t get enough sleep.

Any one of these factors might be an underlying cause of overtime-related heart problems, but none of them fully explains the study results, said Marianna Virtanen, an epidemiologist at the Finnish Institute of Occupational Health in Helsinki and University College London.

Future research will explore the reasons why overtime is bad for your heart. In the meantime, it can’t hurt to relax a little more at work and get a good night’s sleep.

Click here for a video discussion about how to prevent heart disease.