Working long hours increases the risk of developing a dangerous irregular heartbeat, research has shown.
Scientists compared people who work 35 to 40 hours per week with hard grafters who labour for 55 hours or more.
Over a period of 10 years, the harder workers were 40% more likely to suffer the condition, known as atrial fibrillation (AF).
Statistical analysis showed that for every 1,000 participants, an extra 5.2 cases of AF occurred among those working long hours.
Results from the study of 85,500 men and women from the UK, Denmark, Sweden and Finland are reported in the European Heart Journal.
Lead researcher Professor Mika Kivimaki, from University College London, said: "These findings show that long working hours are associated with an increased risk of atrial fibrillation, the most common cardiac arrhythmia.
"This could be one of the mechanisms that explain the previously observed increased risk of stroke among those working long hours.
"Atrial fibrillation is known to contribute to the development of stroke, but also other adverse health outcomes, such as heart failure and stroke-related dementia."
Atrial fibrillation raises the risk of having a stroke because it causes blood to pool and form clots.
During the 10-year follow-up period, a total of 1,061 new cases of AF were recorded among the study participants.
Prof Kivimaki said: "Those who worked long hours had a 1.4 times higher risk of developing atrial fibrillation, even after we had adjusted for factors that could affect the risk, such as age, sex, socio-economic status, obesity, leisure time physical activity, smoking and risky alcohol use.
"Nine out of 10 of the atrial fibrillation cases occurred in people who were free of pre-existing or concurrent cardiovascular disease.
"This suggests the increased risk is likely to reflect the effect of long working hours rather than the effect of any pre-existing or concurrent cardiovascular disease, but further research is needed to understand the mechanisms involved."
He added: "A 40% increased extra risk is an important hazard for people who already have a high overall risk of cardiovascular disease due to other risk factors such as older age, male sex, diabetes, high blood pressure, high cholesterol, overweight, smoking and physical inactivity, or living with an established cardiovascular disease.
"For a healthy, young person, with few if any of these risk factors, the absolute increased risk of atrial fibrillation associated with long working hours is small."
Professor Sir Nilesh Samani, medical director of the British Heart Foundation, said: "Atrial fibrillation is a common heart rhythm disorder that affects over one million people in the UK and which increases the risk of stroke.
"Although we know some of the causes of atrial fibrillation, such as age, high blood pressure, heart valve disease and excess alcohol consumption, many patients develop the condition without an obvious cause.
"The suggestion that longer working hours may be a cause of atrial fibrillation is very interesting.
"Significantly, this study clearly shows that the link between atrial fibrillation and long working hours has nothing to do with the other, already known, risk factors for the condition.
"However, the observational nature of this research means these findings cannot confirm the cause of this relationship - it could be long working hours, it could be the type of work people do or it could be some other, unmeasured, factor.
"More research is needed to understand and prove what's behind this association.
"Only then can we look at our recommendations."