“Working more than eight hours a day raises the risk of heart disease by 80%,” reported the Daily Mail, while The Sun said “overtime’s a killer…literally”.
The news is based on a study that pooled the results of previous studies looking at the association between “longer working hours” and coronary heart disease (CHD). Those working longer hours were shown to be 80% more at risk of CHD.
However, there were significant inconsistencies between the studies that cast serious doubt on the validity of any conclusion about a link between CHD and working hours. These inconsistencies included the definitions of “longer working hours” (from 40 to 65 hours a week).
The studies were also inconsistent in their type, making the overall pooling inappropriate. When the researchers removed less well-designed studies from their analysis, the estimate was lower; in the region of 40% increased risk.
Finally, as only one of the studies was from the UK, the findings may not be applicable to workers in this country.
This study suggests that those who work longer hours may have an increased risk of CHD, but stops well short of proving that one causes the other. There are many other factors that may influence this association.
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Where did the story come from?
The study was carried out by researchers from the Finnish Institute of occupational health and was funded by numerous charity and academic institutions, including the British Heart Foundation and the Medical Research Council. No conflicts of interest were declared by the study authors.
The study was published in the peer-reviewedAmerican Journal of Epidemiology.
The Mail’s headline, “Working more than eight hours a day raises the risk of heart disease by 80%,” was misleading and inaccurate. The study’s 80% figure related to a range of definitions of long working hours, only some of which defined this as more than eight hours a day (a 40-hour, 5-day week). Others defined this as significantly more (more than 65 hours a week).
However, The Sun pointed out that the risk found in this study could be between 40% and 80%.
What kind of research was this?
A systematic review and meta-analysis is an effective way of pooling the findings of multiple studies aiming to answer similar research questions into one summarised result.
The quality of the systematic review and meta-analysis is inherently dependent on the quality of the studies it includes. How thorough it has been in identifying all the relevant research literature to include in the first place is also important.
The researchers hypothesised that people working longer hours are more likely to be exposed to high job demands and to have less time for recreational activities and exercise than their counterparts who work fewer hours. Consequently, long working hours may be associated with CHD events such as heart attacks and angina. They reported that CHD is currently a leading cause of death, and that projections indicated that this would continue for the next several decades.
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What did the research involve?
The researchers searched two medical research databases for studies examining the association between long working hours and CHD. To find further studies they scrutinised the reference sections of the papers identified for further relevant research as well as contacting four experts in the field.
Studies included in the systematic review had to be primary peer-reviewed research of one of the following:
- a cross-sectional study, where data is collected at one point in time
- a case-control study, where people with a certain medical condition are compared with a control group without it in order to identify possible risk factors
- a prospective cohort study, where people are followed over time to see how different factors affect their health outcomes
They also had to report on the association between working hours (self-reported or register-based) and CHD (self-reported, clinically validated or register-based).
No standard definition of “long working hours” was used by the researchers and the definition varied between included studies. For example, some reported overtime, some used more than 10 hours of work a day, some more than 40 hours a week and others more than 65 hours a week.
Data from all included studies was extracted and pooled using meta-analysis to give the association between working hours and CHD risk. Two analyses were conducted. The first pooled results adjusting for age and sex, and where possible socioeconomic position (minimally adjusted).
The second (maximally adjusted) took into account more potentially influential factors, including:
- study location
- study design
- follow-up time
- number of participants
- number/percentage of men
- distribution of the study sample by age and socioeconomic position
- methods used to measure working hours
- methods used to measure the CHD
The analysis compared the risk of developing CHD in people working “normal” hours with those working longer hours. Definitions of these varied from study to study.
What were the basic results?
A total of 12 studies (seven case-control, four prospective cohort, one cross-sectional) containing 22,518 participants (2,313 CHD cases) were included in the systematic review and meta-analysis. Five studies included Japanese participants, two were from the USA and the rest were European including one from the UK.
The included studies varied in size, study design and how they measured working time and CHD. The pooled results also varied depending on which subgroup of studies was included, or whether all 12 were included.
CHD was recorded in a variety of ways across different studies, including:
- first hospital admission due to heart attack alone
- first hospital admission due to heart attack or angina combined
- first and recurrent heart attack events combined
- symptoms of angina
The minimally adjusted meta-analysis of all 12 studies showed that those working longer hours were 80% more at risk of CHD than those not working long hours (relative risk ratio of 1.8, 95% conﬁdence interval 1.42 to 2.29). Seven of the included studies found a statistically significant link between long working hours and a higher risk of CHD while five studies found the same link, but it was not statistically significant.
Maximally adjusted analysis showed that those working longer hours were 59% more at risk of CHD than those not working long hours (relative risk ratio 1.59, 95% conﬁdence interval 1.23 to 2.07).
When the analysis was restricted to the four prospective studies the increase in relative risk was lower still, at 39% (relative risk ratio 1.69 95% conﬁdence interval 1.12 to 1.72).
How did the researchers interpret the results?
The authors concluded that “results from prospective observational studies suggest an approximately 40% excess risk of CHD in employees working long hours”. They possibly singled out the prospective studies as these give a better indication of cause and effect than the other study types included (cross-sectional or case control studies), representing the strongest evidence included in the review.
This systematic review and meta-analysis of 12 observational studies showed that longer working hours (varying definitions used) may be associated with an increased relative risk of CHD. The adjusted pooled results from the best studies suggested those working longer hours were 40% more at risk of CHD compared with those who didn’t.
The main limitation of this review is the variation in the studies it included. This calls into question how useful it was to pool all the results from studies that were so different, and what can be realistically interpreted from the pooled findings.
Due to the large variation in study characteristics, particularly the way they measured longer working hours and CHD, the overall pooled risk figure (80% increased risk) is not particularly informative. For example, it is not possible to say how many hours count as “longer working hours” as the definitions varied among the studies reviewed, from 10 or more hours a day to more than 65 hours a week.
Seven were case-control studies where researchers look at the medical history of people with heart disease. The evidence provided by case-control studies can be distorted by recall bias as people with heart disease are more likely to recall being overworked than healthy people.
Similarly, of the 12 studies only one was conducted in the UK. As working habits vary between countries this may limit how relevant these findings are to UK workers.
Lead researcher Dr Marianna Virtanen is quoted as suggesting that the increase in risk could be due to “prolonged exposure to stress”. She added that “other triggers could be poor eating habits and lack of exercise due to restricted leisure time”.
This study suggests that those who work longer hours may have an increased risk of CHD but stops well short of proving this through causal evidence. Similarly, this study does not tell us how many hours are too many, or how many hours are detrimental to health.
- Virtanen M, Heikkilä K, Jokela M, et al. Long working hours and coronary heart disease: a systematic review and meta-analysis (PDF, 347.7kb). American Journal of Epidemiology. Published online September 5 2012