Burning the candle at both ends might impress your boss, but you could be sacrificing your health in the process.

A study published in The Lancet on Wednesday finds a strong connection between people who work 55 or more hours per week and cardiovascular disease. Those who work such long hours were found to have a 33% increased risk of stroke and 13% greater chance of developing coronary heart disease compared to people who work the standard 35- to 40-hour work week.

Researchers from University College London reviewed 42 studies of hundreds of thousands of men and women from Europe, the U.S., and Australia for several years. Their results held even after controlling for demographic factors—age, sex, socioeconomic status—and health behaviors—like smoking, alcohol consumption, and physical activity.

Working overtime in general, even if it’s not the 55 hour maximum the group studied, also affects health outcomes negatively. Working between 41 to 48 hours led to a 10% increased risk of stroke and upping work hours to between 49 and 54 hours a week caused a 27% increased risk of stroke.

The authors aren’t sure exactly what the link is, but noted a few potential causes. For one, working long hours tends to be correlated with risky health behaviors, like drinking more alcohol or sitting for hours at a time. Those behaviors, combined with the stress associated with working overtime, could be a perfect recipe for a stroke or cardiovascular strain.

Read next: Want a Four-Day Workweek? Here’s How to Make it Happen

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Write to Tanya Basu at tanya.basu@time.com.