People who work three hours or more of overtime per day run a 60-per cent higher risk of bad heart problems compared to those who work regular hours, a study published in the European Heart Journal said.
The findings came from a long-term research project into 6,014 British civil servants aged 39-61, two-thirds of whom were men, who had healthy hearts at the start of the probe in the early 1990s.
At an 11-year monitoring point, 369 of the volunteers had either died of coronary heart disease (CHD) or had had a non-fatal heart attack or angina.
After accounting for risk factors such as smoking, overweight and high cholesterol, doctors found that working between three and four hours of overtime each day was associated with a 60 per cent greater risk compared to those who did no overtime.
Those who worked overtime tended to be slightly younger than the non-overtime group, were likelier to be men rather than women and be in a higher occupational grade.
The investigators say the link between cardiac problems and overtime seems clear but the cause is less so, given the complexities of heart disease and relationships in the workplace.
For instance, working overtime could affect metabolism or mask depression, anxiety and sleeplessness, they caution.
There is also a phenomenon called "sickness presenteeism", whereby employees who work overtime are likelier to work while ill, ignore symptoms and not seek medical health.
In addition, people who like their job and have greater latitude in work decisions tend to work longer hours just for the pleasure, and may have a lower rate of CHD.
"More research is needed before we can be confident that overtime work would cause CHD," said Marianna Virtanen, an epidemiologist at the Finnish Institute of Occupational Health and University College London.