People who worked long hours had a higher risk of stroke, especially if they worked those hours for 10 years or more, according to new research in the American Heart Association’s journal Stroke.
Researchers reviewed data from CONSTANCES, a French population-based study group started in 2012, for information on age (18-69), sex, smoking and work hours derived from questionnaires from 143,592 participants.
Cardiovascular risk factors and previous stroke occurrences were noted from separate medical interviews.
- overall 1,224 of the participants, suffered strokes;
- 29% or 42,542, reported working long hours;
- 10% or 14,481, reported working long hours for 10 years or more; and
- participants working long hours had a 29% greater risk of stroke, and those working long hours for 10 years or more had a 45% greater risk of stroke.
Long work hours were defined as working more than 10 hours for at least 50 days per year.
Part-time workers and those who suffered strokes before working long hours were excluded from the study.
“The association between 10 years of long work hours and stroke seemed stronger for people under the age of 50,” said study author Alexis Descatha, M.D., Ph.D., a researcher at Paris Hospital, Versailles and Angers University and at the French National Institute of Health and Medical Research (Inserm).
“This was unexpected. Further research is needed to explore this finding.
“I would also emphasize that many healthcare providers work much more than the definition of long working hours and may also be at higher risk of stroke,” Descatha said.
“As a clinician, I will advise my patients to work more efficiently and plan to follow my own advice.”
Previous studies noted a smaller effect of long work hours among business owners, CEOs, farmers, professionals and managers.
Researchers noted that it might be because those groups generally have greater decision latitude than other workers.
In addition, other studies have suggested that irregular shifts, night work and job strain may be responsible for unhealthy work conditions.
In the early 1800s, the industrial working week was 14 to 16 h a day, 6 days a week.
Since then, enormous reductions in working hours have taken place as a result of increased efficiency and productivity, collective bargaining mainly via trade unions, and progressive legislation .
However, in modern society, working time is no longer limited to hours spent at the workplace. In many occupations, work can be done at any time and in any place.
An increasingly common opinion is that high demands at work result in insufficient time to get work done within a standard 7- to 8-h workday.
For low-wage blue-collar employees, long working hours may comprise two or more contemporaneous part-time jobs.
Globally, the longest annual average working hours are those in Mexico, Costa Rica, and South Korea , although a work schedule that has become pervasive in Chinese companies is commonly referred to as ‘996’: working from nine in the morning to nine in the evening, 6 days a week .
In Europe, the average number of working hours seems to be decreasing. However, a detailed analysis of extreme working hours shows polarization with an increasing proportion of the workforce both working very long hours and short hours .
This specific pattern was observed at least in Europe and North America.
In this review, we summarize the evidence provided by prospective studies on long working hours and cardiovascular disease (CVD), the leading cause of death globally.
We also include studies that have addressed potential mechanisms linking long working hours with CVD risk and discuss limitations in the present evidence, prospects for future studies, and implications for clinical practice.Go to:
Long Working Hours and Cardiovascular Disease: The Current Evidence
Particular attention has been paid to cardiovascular diseases (CVD) stemming from the observation in Japan of ‘karoshi’—death from overwork .
However, to assess causation, the most convincing evidence should come from randomized controlled trials.
Regarding CVD, we are not aware of any studies that have randomized participants in terms of working hours to assess the effects on CVD incidence or progression.
Therefore, the evidence available relies on observational data with known limitations, as will be discussed later.
The first systematic review and meta-analysis of observational studies on the association with coronary heart disease (CHD) was published in 2012 .
The meta-analysis included only published studies (n = 12), of which 7 were case-control studies, 4 prospective, and 1 a cross-sectional study.
This suggested an overall relative risk of 1.59 (95% CI 1.23–2.07) associated with long working hours.
An analysis restricted to prospective studies found a relative risk of 1.39 (95% CI 1.12–1.72) while the case-control studies indicated an odds ratio of 2.43 (95% CI 1.81–3.26) for long working hours.
The authors considered a major limitation among the studies to be the inconsistent assessment of exposure (long working hours) as well as problems related to publication bias and case-control design (recall bias among CHD cases).
In the Individual-Participant-Data Meta-analysis in Working Populations (IPD-Work) Consortium [17,18], these limitations were addressed by collecting both published and unpublished data from prospective cohort studies and by carefully harmonizing the exposure (long working hours) and cardiovascular outcomes to be as consistent across studies as possible.
The IPD-Work Consortium had already published a meta-analysis on perceived work stress and CHD in 2012 .
An individual participant meta-analysis on long working hours and the incidence of CHD and stroke from IPD-Work was published in 2015 .
It included studies from the USA, Europe (the UK, Northern Ireland, Germany, Belgium, the Netherlands, Denmark, Sweden, Finland), Israel, and Australia.
In 22 cohort studies and 598,470 participants for the analysis of CHD and 14 cohort studies and 520,925 participants for the analysis of stroke, hazard ratios for working 55 h or more a week, compared to a standard 35–40 h working week, were 1.13 (95% CI 1.02–1.26) for CHD and 1.33 (95% CI 1.11–1.61) for stroke.
A dose-response relationship (increasing risk associated with increasing working hours in full-time employees) was found for stroke but not for CHD.
Sub-group analyses, multivariable adjustment for other risk factors, and analyses stratified by the method of stroke ascertainment suggested that the excess risk of stroke was robust.
Reverse causality bias was addressed by excluding cases that occurred during the first 3 years of follow-up—no evidence was found to suggest that the association was attributable to reverse causation.
Since the IPD-Work meta-analysis was published, at least two independent large-scale studies have examined the association between long working hours and cardiovascular disease.
In these studies, responses of 145,861 to 199,035 employees to the Danish Labour Force Survey in 1999 to 2013 were linked to records of hospitalizations and deaths from national registers until 2014 [19•,20•]. With 35–40 working hours per week as reference, the estimated rate ratio for working ≥ 55 h per week was 0.89 (95% CI 0.69–1.16) for overall stroke, 0.86 (95% CI 0.61–1.22) for ischemic stroke, and 1.33 (95% CI 0.82–2.15) for hemorrhagic stroke [19•].
The rate ratio of ischemic heart disease for > 48 compared with 32–40 weekly working hours was 1.09 (95% CI 0.96–1.24 among participants without a recorded heart disease 5 years before the survey (the rate ratio was not reported for ≥ 55 weekly working hours) [20•].
Figure Figure11 shows the current state of evidence on the association between long working hours and the onset of CHD and stroke, in which the results from the previous systematic review and individual-participant data meta-analysis by IPD-Work published in 2015, which included 22 studies on CHD and 14 studies on stroke , are supplemented with findings from the Danish Labour Force Survey [19,20], which were identified in our literature search of PubMed in June 30, 2018.
The Danish studies are based on the same population-based cohort from Denmark, with an average follow-up of 7.7 years.
For the CHD meta-analysis, we used the estimate in which all CHD cases 5 years before the survey were excluded instead of 1 year, as was done in the main analysis of that study .
We used random effects meta-analysis (Stata 15.1) to obtain new relative risk estimates for CHD and stroke, including all original estmates from 22 CHD studies and 14 stroke studies in the analyses, to which we added the estimates reported in the Danish studies.
As shown in Fig. 1, combining the findings from IPD-Work and these new studies in a random effects meta-analysis led to a relative risk of 1.12 (95% CI 1.03–1.21) for CHD and 1.21 (95% CI 1.01–1.45) for stroke.
The estimates for coronary heart disease are similar for both studies, but the point estimates of stroke from IPD-Work and the Danish Labour Force Survey appear to differ although the I2 statistics suggested no significant heterogeneity between the 23 and 15 meta-analyzed studies (I2 = 0.0%, p = 0.54 for CHD; I2 = 15.2%, p = 0.28 for stroke).
There were methodological differences between IPD-Work and the Danish study. Stroke incidence, for example, was lower in IPD-Work (4.5 per 10,000 person-years) than in the Danish Survey (14.3 per 10,000 person-years), which might point to diagnostic differences being relevant.
In summary, the total data available from observational studies suggest small associations between long working hours and CVD outcomes. The associations seemed not to be confounded by known CVD risk factors, such as health behaviors.
More information: Marc Fadel et al, Association Between Reported Long Working Hours and History of Stroke in the CONSTANCES Cohort, Stroke(2019). DOI: 10.1161/STROKEAHA.119.025454
Journal information: Stroke
Provided by American Heart Association