Working 55 or more hours a week is associated with an increased risk for stroke, and the more hours put in at the office or other workplace the greater the increase in risk, a new meta-analysis shows.
Long working hours were also associated with an increased risk for coronary heart disease (CHD), but the association was weaker than that for stroke, the results suggest.
"Our study is important because it shows, for the first time, that individuals who work long hours may be at an increased risk of stroke," said Mika Kivimäki, PhD, professor, Department of Epidemiology and Public Health, University College London, United Kingdom.
"These findings suggest that more attention should be paid to the management of vascular risk factors in individuals who work long hours," the authors conclude.
The study was published online August 20 in The Lancet.
Researchers searched PubMed and Embase for published studies and scrutinized all relevant major reviews. They also included unpublished data from Individual Participant-Data Meta-analysis in Working populations (IPD-Work), a consortium of prospective cohort studies.
They excluded disease events that took place in the first years of follow-up to reduce bias due to reverse causation.
The final analysis, which included 25 studies from the United States, Australia, Finland, Denmark, Sweden, the Netherlands, Belgium, Germany, the United Kingdom, Northern Ireland, and Israel, was larger than any previous one. For example, for the association between long working hours and CHD, it included 603,838 men and women.
"The five previous published studies on long working hours and coronary heart disease comprised in total 15,000 participants," said Dr Kivimäki. "Our study is 40 times larger."
And for the association between long working hours and stroke, 528,908 men and women contributed to the analysis. While a previous study from Northern Ireland examined this association, the current analysis included 17 studies from several European countries and the United States.
"This means that we were able to explore the links to coronary heart disease and stroke with a much more comprehensive evidence base than has previously been possible," said Dr Kivimäki.
During the 7- to 8-year follow-up, 4768 participants had a coronary event and 1722 had a stroke.
Compared with standard working hours (35 to 40 hours per week), working long hours, defined as 55 or more hours a week, raised the risk for stroke (relative risk [RR], 1.33; 95% confidence interval [CI], 1.11 - 1.61; P = .002) after adjustment for age, sex, and socioeconomic status. The risk remained the same for different methods of determining stroke.
The analysis also uncovered a dose response: the RRs for 41 to 48, 49 to 54, and 55 or more hours were 1.10 (95% CI, 0.94 - 1.28), 1.27 (95% CI, 1.03 - 1.56), and 1.33 (95% CI, 1.11 - 1.61), respectively (P < .0001).
"The stroke findings were surprisingly robust," commented Dr Kivimäki.
As for the association between long working hours and CHD, the RR was 1.13 (95% CI, 1.02 - 1.26; P = .02), a "modest overall increase," the authors note. The authors found no differences in results between European and US studies but were unable to make comparisons between individual countries, said Dr Kivimäki. The analysis also did not include any studies from Japan, whose workforce has something of a reputation for working exceedingly long hours, he noted.
Researchers aren't certain what underlying mechanisms are driving the link between working long hours and stroke risk. Sudden death from a stroke is believed to result from a repetitive triggering of the stress response.
It's possible, said Dr Kivimäki, that higher alcohol consumption is a contributing factor, although the association remained even after adjustment for alcohol use in the analysis. As well, those who work long hours might ignore symptoms of disease, he said.
Dr Kivimäki and his colleagues are examining several potential mediating mechanisms. "At the earlier stages of the disease process, these include stress and sedentary behavior, for example, extended sitting, and, in later stages, established risk factors for stroke, such as cardiac arrhythmias, left ventricular hypertrophy, and the formation of blood clots."
Until cause and effect are determined, "we suggest that prevention proceed along established lines of multifactorial cardiovascular disease risk reduction," said Dr Kivimäki. This includes keeping blood pressure, lipid levels, and blood glucose within the normal range, getting adequate physical activity, following a healthy diet, and avoiding being overweight and stressed.
Dr Kivimäki emphasized that management of vascular risk factors is particularly important among people who work long hours.
Tackling the health issues involved with overwork is "not a black and white" issue, said Dr Kivimäki. Working long hours may be gratifying to employees and may help them reach career goals, which could benefit their family and others around them. But at the same time, it's important that people be aware of the health toll this work load can take.
"In the European Union, we have a working time directive that gives the employee the right to limit weekly working hours to 48 hours on average if he or she wishes. I think this is a useful policy as then that employee can make the choice."
A limitation of the study was that working time was self-reported and measured only once.
In an accompanying editorial, Professor Urban Janlert, Department of Public Health and Clinical Medicine, Division of Epidemiology and Global Health, Umeå University, Sweden, called the study "pioneering" and said its results provide "the strongest indication of a causal association" between long working hours and stroke.
Long working hours are "not a negligible occurrence," said Dr Janlert. Among member countries of the Organization of Economic Cooperation and Development, a mean of 12% of employed men and 5% of employed women work more than 50 hours a week.
Turkey has the highest proportion of workers working more than 50 hours per week (43%) and the Netherlands has the lowest (<1%).
The finding of a significant stroke risk and a less convincing CHD risk among employees working long hour is "interesting" since CHD is more prevalent than stroke among people of working age, said Dr Janlert.
However, as Dr Kivimäki pointed out, one in four stroke events occurs before age 65 years.
The new study results could be tested by randomly allocating some employees working long hours to working less and measuring variables such as stress response, blood pressure, salt intake, long exposure to sedentary positions, and sleep time, concluded Dr Janlert. "Such investigations would be beneficial because the consequences of long working hours, rather than long working hours alone, are likely to be the underlying causes" of the current findings.
The study received funding from the Medical Research Council, Economic and Social Research Council, European Union New and Emerging Risks in Occupational Safety and Health Research Programme, Finnish Work Environment Fund, Swedish Research Council for Working Life and Social Research, German Social Accident Insurance, Danish National Research Centre for the Working Environment, Academy of Finland, Ministry of Social Affairs and Employment (Netherlands), US National Institutes of Health, and British Heart Foundation. Dr Kivimäki and Dr Janlert have disclosed no relevant financial relationships.
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