People who work long hours -- 10 hours a day -- are more likely to have heart-related problems than those who work less, researchers in Britain report in the European Heart Journal.
The researchers followed more than 6,000 civil servants in the U.K. between 1991 and 2004 for an average of 11 years per person, tabulating how much people reported working over 7 hours a day, and their health status.
By the end of the study, people who had worked 10-hour days were 60 percent more likely to have heart problems than people who worked seven-hour days. People who worked 8 or 9 hours a day had no significant increases in heart problems.
In all, the researchers found 396 heart problems in the group, which included death due to heart disease, non-fatal heart attacks and angina.
"I see this as an important new finding from a well-established and trustworthy research group," Dr. Redford Williams, head of behavioral medicine at Duke University in Durham, N.C., told ABCNews.com.
"It's independent of all the usual suspects, including not only blood pressure, cholesterol and overweight, but Type A behavior, not enough sleep and depression," he said.
The researchers tried to statistically control for the effects of smoking, high cholesterol and 21 other risk factors that could influence heart health.
After accounting for the most obvious heart risks, they still found the association between working long hours and heart problems.
But the study only showed an association between working long hours and heart problems.
Researchers say it would take more work to find a cause, and how overtime might hurt a person's heart.
Researchers Can't Say How Working Overtime May Hurt Hearts
"One plausible explanation for the increased risk could be that adverse lifestyle or risk factor changes are more common among those who work excessive hours, compared with those working normal hours," said Dr. Marianna Virtanen, an author of the paper and an epidemiologist at the Finnish Institute of Occupational Health in Helsinki and University College London.
Virtanen and her colleagues found people who worked overtime were also more likely to have "type A" personality traits such as being aggressive, competitive and tense. They were also more likely to have depression, anxiety and possibly were not getting enough sleep.
"Another possibility is that the chronic experience of stress adversely affects metabolic processes. It is important that these hypotheses should be examined in detail in the future," Virtanen said in a press release.
But in the meantime, some still think people should be wary of potential health problems from working overtime.
"If the effect is truly causal, the importance is much greater than commonly recognized," Gordon McInnes, Professor of Clinical Pharmacology at the University of Glasgow's Western Infirmary in Scotland wrote in an accompanying editorial published in the European Heart Journal. "Overtime-induced work stress might contribute to a substantial proportion of cardiovascular disease."
"Physicians should be aware of the risks of overtime and take seriously symptoms such as chest pain, monitor and treat recognized cardiovascular risk factors, particularly blood pressure, and advise an appropriate lifestyle modification," McInnes wrote.
In the article, the authors point out that people in Britain tend to put in more overtime than others elsewhere in the European Union.
But the United States ranks sixth for overtime, according to a 2004 survey by the Organization for Economic Cooperation and Development. Other countries with high rates of overtime include South Korea, Greece, Mexico, Australia and Japan.
Williams said given the current economic climate, it will be hard for U.S. workers to say "no thanks" to overtime for the sake of their health.
"It may be hard, given the economic situation's effects on working conditions, to reduce overtime," said Williams. "It could be interesting to see if training in stress-coping skills that have been shown to reduce not only psychosocial risk factors like anger, depression and perceived stress, but also blood pressure and heart rate surges when recalling a situation that made one angry, would ameliorate the health-damaging effects of overtime."