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People who work 55 hours or more per week have a 33 percent greater risk of stroke and a 13 percent greater risk of coronary heart disease than those working standard hours, researchers reported on Wednesday in the Lancet.

The new analysis includes data on more than 600,000 individuals in Europe, the United States and Australia, and is the largest study thus far of the relationship between working hours and cardiovascular health. But the analysis was not designed to draw conclusions about what caused the increased risk and could not account for all relevant confounding factors.

“Earlier studies have pointed to heart attacks as a risk of long working hours, but not stroke,” said Dr. Urban Janlert, a professor of public health at Umea University in Sweden, who wrote an accompanying editorial. “That’s surprising.”

Mika Kivimaki, a professor of epidemiology at University College London, and his colleagues combined the results of multiple studies and tried to account for factors that might skew the results. In addition to culling data from published studies, the researchers also compiled unpublished information from public databases and asked authors of previous work for additional data.

Dr. Steven Nissen, the chief of cardiovascular medicine at the Cleveland Clinic, found the methodology unconvincing. “It’s based upon exclusively observational studies, many of which were unpublished,” and some never peer-reviewed, he said.

Seventeen studies of stroke included 528,908 men and women who were tracked on average 7.2 years. Some 1,722 nonfatal and deadly strokes were recorded. After controlling for smoking, physical activity and high blood pressure and cholesterol, the researchers found a one-third greater risk of stroke among those workers who reported logging 55 or more hours weekly, compared with those who reported working the standard 35 to 40 hours.

While the increase in risk for each individual was slight, experts said the effect was noteworthy in a large population in which many people are working long hours.

In his editorial, Dr. Janlert said, “Long working hours are not a negligible occurrence.” Full-time American workers in nonagricultural industries labor for an average 42.5 hours per week, according to Bureau of Labor Statistics.

But workweek hours vary sharply depending on occupation and company culture. In Gallup surveys in 2013 and 2014, nearly four in 10 full-time American workers reported laboring at least 50 hours weekly, and half said they usually work more than 40 hours.

Dr. Kivimaki and his colleagues also found the risk of stroke increased as work hours lengthened. But he said, “we found no differences between men and women, or between older people and younger ones, or those with higher or lower socioeconomic status.”

Dr. Ralph Sacco, a former president of the American Heart Association, said, “The consistency of the findings across published and unpublished data — that alignment is a strength and makes this more convincing.”

The analysis of coronary heart disease among workers included 25 studies involving 603,838 people. (Some of these studies were also used in the stroke analysis.) After a mean of 8.5 years, 4,768 had received diagnoses of heart disease as a cause of death or hospitalization. The researchers accounted for age, sex and socioeconomic status.

Dr. Stephen L. Kopecky, a professor of medicine in the division of cardiology at the Mayo Clinic, said that this analysis did not fully account for the effects of cholesterol, family history and blood pressure in all cases, so it is possible that long hours are not entirely to blame.

Furthermore, he argued, it matters to stroke risk whether an employee has a job with high demands and little control, which researchers call “job strain.”

“You have higher blood pressure when you have job strain,” Dr. Kopecky said. “And guess what that’s associated with? Stroke.”

Another limitation: individuals were asked just once the number of hours they work.

A randomized trial could shed light on whether working long hours actually causes strokes. For example, scientists could assign some people working long hours to reduce them, and other overachievers to keep up their long hours, and then measure the health consequences and track factors like blood pressure, sleep duration, and stress response.

Dr. Sacco, chairman of neurology at the University of Miami Miller School of Medicine, said a randomized study where “we can measure all the unmeasured factors” would help confirm these new results.

”It’s not a practical trial to do,” he said.

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