Working more than 55 hours a week is associated with a 33 percent increased risk of stroke and a smaller, but still significant, 13 percent increased risk in coronary heart disease, according to new research published Wednesday in the journal The Lancet.
That’s not good news for American workers. According to a 2014 Gallup poll, adults employed full time in the U.S. put in an average of 47 hours a week at their jobs. In fact, about 40 percent report logging in for at least 50 hours a week, and 18 percent say they work 60 or more hours a week.
The new research — a meta-analysis — scrutinized published and unpublished data from 25 previous studies that had looked at the relationship between working hours and cardiovascular health in the United States, Australia and several European countries.
Its authors — a team of international researchers — say their analysis is the largest study on this topic conducted so far.
Link strongest for stroke
The data used in the analysis on the relationship between working hours and stroke came from 17 of the 25 studies and involved 528,909 men and women who were followed for an average of 7.2 years. When they entered these studies, the participants had no history or symptoms of heart disease. During the course of the studies, 1,722 experienced a stroke.
The meta-analysis revealed that the people who worked 55 hours or more per week were 33 percent more likely to have a stroke than those who worked a more “standard” week of 35 to 40 hours. That association held even after adjusting for such factors as smoking, physical activity, high blood pressure and cholesterol.
The association was the same for both genders and across all age and socioeconomic levels.
The analysis also revealed that the stroke risk increased as people’s working hours climbed higher. Working up to 48 hours a week was associated with a 10 percent increased risk in stroke, while working up to 54 hours increased the risk by 27 percent — slightly less than the 33 percent increased risk observed among the employees working 55 hours or more.
Smaller risk for CHD
To analyze the relationship between long working hours and heart disease, the researchers looked at data from all 25 studies, which involved 603,838 men and women in the United States, Australia and Europe. They were followed for an average of 8.5 years, during which time 4,768 experienced some kind of coronary heart disease “event” — a diagnosis of the disease, for example, or hospitalization for a stroke or heart attack, or death.
The analysis found that people who worked 55 or more hours a week were 13 percent more likely to develop coronary heart disease during the years they were being observed than their peers who worked 35 to 40 hours a week. That association also held when other risk factors for heart disease were taken into account.
Interestingly, however, the risk for coronary heart disease was greater among long-hour workers in lower-paying jobs than among those in higher-paying ones.
This meta-analysis wasn’t designed to identify the factors that might explain the association between long working hours and an increased risk of stroke and heart disease. The authors note, however, that previous research has suggested that sudden death from overwork may be the result of “a repetitive triggering of the stress response.”
They also point to evidence that has found a possible link between an increased risk of stroke and sitting for long periods at work. Also, people who work long hours are “slightly more prone to risky drinking than those who work standard hours,” they write, and are more likely to ignore symptoms of heart disease, and thus often delay seeking medical care.
Caveats and limitations
Of course, this was an observational study, which means it cannot prove cause and effect. Other factors, not included in the analysis, could explain the results.
Furthermore, the participants self-reported their working hours. They could have exaggerated — or underestimated — the amount of time they dedicated to their jobs each week. Also, they reported their hours only once. Those hours could have changed over the period they were in the studies.
In addition, the risks described in this meta-analysis are relative, and, as risks go, a 33 percent increase in relative risk is slight.
In talking with the BBC, one of the study’s authors put the findings in what may be a more helpful perspective: Among the people working 35 to 40 hours a week, the data revealed fewer than five strokes per 1,000 employees per decade. That number increased to six strokes per 1,000 employees per decade among the people working 55 hours or more.
This is not a huge difference, say the study’s authors, but still a significant one given how many people are now working longer hours.
You can download and read the study in full on The Lancet website.