Individuals who worked at least 55 hours a week were about 33% more likely to have a stroke than were those who worked 35-40 hours, according to a large meta-analysis of prospective studies.
The finding is “robust,” wrote Mika Kivimäki, Ph.D., of University College London and his associates. “There was no evidence of between-study heterogeneity, reverse causation bias, or confounding. Furthermore, the association did not vary between men and women or by geographical region, and was independent of the method of stroke ascertainment” (Lancet 2015;386[10005]:1739-46).
“Long working hours were also associated with incident coronary heart disease, but this association was weaker than that for stroke,” they noted.
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Past studies have linked long working hours to cardiovascular disease, but prospective data are “scarce, imprecise, and mostly limited to coronary heart disease,” the researchers wrote. To assess relationships between working hours, incident CHD, and stroke, they searched Medline and Embase for studies published through August 2014, and also acquired unpublished data from 20 cohort studies through the Individual-Participant-Data Meta-analysis in Working Populations (IPD-Work) Consortium and open-access databases. These efforts yielded 25 total studies of 24 cohorts in the United States, Europe, and Australia, comprising CHD risk data for almost 604,000 individuals and stroke risk data for almost 529,000, the researchers said.
The studies reported 1,722 strokes and 4,768 new episodes of CHD after an average of 7.2 and 8.5 years of follow-up, respectively, the investigators determined. After accounting for age, sex, and socioeconomic status, individuals who averaged a 55-hour work week were 33% more likely to have a stroke (relative risk, 1.33; 95% confidence interval, 1.11-1.61; P = .002) and a 13% greater risk of CHD (RR, 1.13; 95% CI, 1.02-1.26; P = .02) than were those who worked 35-40 hours per week.
“We recorded a dose-response association for stroke, with RR estimates of 1.10 (95% CI, 0.94 to 1.28; P = .24) for 41 to 48 working hours; 1.27 (1.03 to 1.56; P = .03) for 49 to 54 working hour; and 1.33 (1.11 to 1.61; P = .002) for 55 working hours or more per week, compared with standard working hours (P less than .0001),” the researchers wrote. Based on the findings, “more attention should be paid to the management of vascular risk factors in individuals who work long hours,” they concluded.
The study was funded by the U.K. Medical Research Council, the Economic and Social Research Council, the European Union New and Emerging Risks in Occupational Safety and Health research program, the Finnish Work Environment Fund, the Swedish Research Council for Working Life and Social Research, German Social Accident Insurance, the Danish National Research Centre for the Working Environment, the Academy of Finland, the Ministry of Social Affairs and Employment (Netherlands), the U.S. National Institutes of Health, and the British Heart Foundation. The investigators reported having no conflicts of interest.