Longer working hours link to stroke risk

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Working 55 hours or more per week is linked to a 33% greater risk of stroke and a more modest (13%) increased risk of developing coronary heart disease compared with working a standard 35 to 40 hour week, according to the largest study in this field so far, led by University College London (UCL).

Professor Mika Kivimäki (UCL department of epidemiology & public health) and colleagues did a systematic review and meta-analysis of published studies and unpublished individual-level data examining the effects of longer working hours on cardiovascular disease up to 20 August, 2014.

Analysis of data from 25 studies involving 603,838 men and women from Europe, the US, and Australia who were followed for an average of 8.5 years, found a 13% increased risk of incident coronary heart disease (a new diagnosis, hospitalisation, or death) in people working 55 hours or more per week compared with those putting in a normal 35 to 40 hour week, even after taking into account risk factors including age, sex, and socio-economic status.

Analysis of data from 17 studies involving 528,908 men and women who were followed up for an average of 7.2 years, found a 1.3 times higher risk of stroke in individuals working 55 hours or more a week compared with those working standard hours. This association remained even after taking into account health behaviours such as smoking, alcohol consumption, and physical activity, and standard cardiovascular risk factors including high blood pressure and high cholesterol.

Importantly, the researchers found that the longer people worked, the higher their chances of a stroke. For example, compared with people who worked standard hours, those working between 41 and 48 hours had a 10% higher risk of stroke, and those working 49 to 54 hours had a 27% increased risk of stroke.

Although the causal mechanisms of these relationships need to be better understood, the authors suggest that increasing health-risk behaviours, such as physical inactivity and high alcohol consumption, as well as repetitive triggering of the stress response, might increase the risk of stroke.

According to Kivimäki: "The pooling of all available studies on this topic allowed us to investigate the association between working hours and cardiovascular disease risk with greater precision than has previously been possible. Health professionals should be aware that working long hours is associated with a significantly increased risk of stroke, and perhaps also coronary heart disease."

Summary
Background: Long working hours might increase the risk of cardiovascular disease, but prospective evidence is scarce, imprecise, and mostly limited to coronary heart disease. We aimed to assess long working hours as a risk factor for incident coronary heart disease and stroke.
Methods: We identified published studies through a systematic review of PubMed and Embase from inception to Aug 20, 2014. We obtained unpublished data for 20 cohort studies from the Individual-Participant-Data Meta-analysis in Working Populations (IPD-Work)
Consortium and open-access data archives: We used cumulative random-effects meta-analysis to combine effect estimates from published and unpublished data. Findings: We included 25 studies from 24 cohorts in Europe, the USA, and Australia. The meta-analysis of coronary heart disease comprised data for 603 838 men and women who were free from coronary heart disease at baseline; the meta-analysis of stroke comprised data for 528 908 men and women who were free from stroke at baseline. Follow-up for coronary heart disease was 5•1 million person-years (mean 8•5 years), in which 4768 events were recorded, and for stroke was 3•8 million person-years (mean 7•2 years), in which 1722 events were recorded. In cumulative meta-analysis adjusted for age, sex, and socioeconomic status, compared with standard hours (35–40 h per week), working long hours (≥55 h per week) was associated with an increase in risk of incident coronary heart disease (relative risk [RR] 1•13, 95% CI 1•02–1•26; p=0•02) and incident stroke (1•33, 1•11–1•61; p=0•002). The excess risk of stroke remained unchanged in analyses that addressed reverse causation, multivariable adjustments for other risk factors, and different methods of stroke ascertainment (range of RR estimates 1•30–1•42). We recorded a dose–response association for stroke, with RR estimates of 1•10 (95% CI 0•94–1•28; p=0•24) for 41–48 working hours, 1•27 (1•03–1•56; p=0•03) for 49–54 working hours, and 1•33 (1•11–1•61; p=0•002) for 55 working hours or more per week compared with standard working hours (ptrend)

University College London material The Lancet article summary

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