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.

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.”