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Working long hours 'increases stroke risk'

12:00pm Friday 21st August 2015 content supplied byNHS Choices

"People working long hours are more likely to have a stroke, according to analysis of more than half a million people," BBC News reports.

Researchers pooled the data from 25 previous studies that included more than 600,000 individuals who were free from heart disease or other types of cardiovascular disease at enrolment. 

They found the risk of developing stroke increased by a third in individuals who worked long hours (above 55 hours a week) compared with individuals with traditional 9-5 working hours. The association of long working hours with heart disease was less - only a 13% increase.

This study has several strengths, including its large size and the inclusion of published and unpublished studies, which removes the risk of publication bias. But the study does have limitations.

The idea that overwork can lead to serious illness and even death is not a new one. The Japanese even have a word for it - "Karoshi". But it is very difficult to prove direct cause and effect. 

Although the researchers tried to account for some health and lifestyle factors that may influence risk, it is problematic to pin down working hours as the single direct cause of a health outcome.

Overall, the results of these studies show an association between long working hours and the risk of developing heart disease and stroke.  

Where did the story come from?

The study was carried out by researchers from various academic institutions in the UK, Finland, France, Sweden, Denmark, Belgium, Germany and Netherlands.

It was funded by multiple organisations, including the Medical Research Council, the Economic and Social Research Council, and the European Union New and Emerging Risks in Occupational Safety and Health research programme.

The study was published in the peer-reviewed medical journal, The Lancet.

The study has been widely reported in the UK media. Overall, the results are accurately reported, but some of the strengths and limitations have not been fully explained. 

The BBC quoted one of the researchers, Dr Mika Kivimaki, of University College London, who said: "People need to be extra careful that they still maintain a healthy lifestyle and ensure their blood pressure does not increase."

While Dr Shamim Quadir of The Stroke Association is quoted as saying: "Working long hours can involve sitting for long periods of time, experiencing stress, and leads to less time available to look after yourself."

He added: "We advise that you have regular blood pressure checks. If you're at all concerned about your stroke risk, you should make an appointment with your GP or health professional." 

What kind of research was this?

This was a systematic review combined with a meta-analysis that assessed the impacts of long working hours on coronary heart diseases and stroke.

The researchers say previous reviews have shown an association between long working hours and cardiovascular disease. However, these studies have several limitations, including few available studies, lack of adjustment for potential confounding factors, and difficulty in proving the direction of effect (that long working hours have preceded the health outcome).

In this review the researchers aimed to include both published and unpublished studies to avoid any publication bias. They also aimed to only include studies where people were free from disease at the start of the study, exclude disease events that took place in the first year of follow-up to make sure of the direction of effect, take confounding factors into account, and look at the influence of socioeconomic class.

Overall, systematic reviews are the best way to gather all evidence on a particular topic, and meta-analysis pools the data from many studies to give an overall indication of the effect. However, the reliability of the findings is only as good as the quality of the studies included.  

What did the research involve?

Researchers identified 25 eligible published and unpublished studies assessing the association between cardiovascular disease and long working hours. Out of the 25 studies, the researchers judged 17 of them to be of a high quality.

The studies were from the US, Australia, Finland, Denmark, Sweden, the Netherlands, Belgium, Germany, the UK, Northern Ireland, and Israel.

The definition of long working hours was mostly 55 hours or more, though some studies used 45 hours or more. The average follow-up period for heart disease was 8.5 years and 7.2 years for stroke. For heart disease outcomes, the results of all 25 studies were pooled, including 603,838 men and women who were free from heart disease at study start. The analysis for stroke included 528,908 men and women from 17 studies who had not had stroke at enrolment.  

What were the basic results?

The review found an association between long working hours and the risk of developing heart disease or stroke.

When adjusted for age, sex and socioeconomic status, long working hours of above 55 hours a week were associated with a modest 13% increase in risk of heart disease (relative risk [RR] 1.13, 95% confidence interval [CI] 1.02 to 1.26) compared with standard working hours (35-40 hours a week).

An analysis restricted to high-quality studies showed the increased risk of heart disease associated with long working hours was higher among those of low socioeconomic groups (RR 2.18, 95% CI 1.25 to 3.81) than in the intermediate group (RR 1.22, 95% CI 0.77 to 1.95) or the high socioeconomic group, where in fact there was no significant link (RR 0.87, 95% CI 0.55 to 1.38).

For stroke, long working hours were associated with a third increased risk of developing stroke (RR 1.33, 95% CI 1.11 to 1.61). Overall, for stroke there was evidence of an increase in risk as the number of additional hours worked increased.

People working for 49-54 hours also had increased risk (RR 1.27, 95% CI 1.03 to 1.56) compared with people working standard hours. For those working 41-48 hours there was a suggestion of increased risk, but the link was non-significant (RR 1.10, 95% CI 0.94 to 1.28).

The associations were not reported to be influenced by gender or geographical area. 

How did the researchers interpret the results?

The researchers concluded by saying: "Our meta-analysis shows that employees who work long hours have a higher risk of stroke than those working standard hours; the association with coronary heart disease is weaker."

They added: "Our findings suggest that more attention should be paid to the management of vascular risk factors in individuals who work long hours." 

Conclusion

This systematic review with meta-analysis aimed to assess the association between long working hours and the risk of developing heart disease and stroke. 

Overall, the study found longer working hours above 55 hours a week was linked to a third increased risk of stroke. The link with heart disease was weaker. 

It also found the influence of long hours on heart disease risk was higher for those of lower socioeconomic groups than it was for those of intermediate or high socioeconomic groups.

This study has several strengths. This includes the large overall sample size and the inclusion of both published and unpublished studies, which should reduce the risk of publication bias. The researchers also excluded disease events that took place in the first year of the follow-up period to better assess the direction of effect. They have also taken into account various confounding factors.

However, this study still cannot prove cause and effect. Though the researchers adjusted for some common confounding factors, various hereditary, health and lifestyle factors may influence the risk of heart disease and stroke. It is not possible to single out working hours as the direct cause of these events.

The studies included may also vary in the working populations covered, definitions of working hours, and assessment of outcomes (such as medical records or self-reports). The studies also came from high-income countries. This makes it hard to generalise the findings to all people.

It is sage advice that we should aim to maintain a healthy lifestyle through a balanced diet and regular exercise habits. Free time to relax away from work is essential to wellbeing, so it is quite plausible that work stress through long working hours may have detrimental health effects.

If you're concerned your working hours are taking a toll on your health, discuss your concerns with your manager or human resources representative. For most professions, employment law states you shouldn't be made to work more than 48 hours a week unless you choose to do so.

Summary

"People working long hours are more likely to have a stroke, according to analysis of more than half a million people," BBC News reports. Researchers pooled the data from 25 previous studies that included more than 600,000 individuals.

Links to Headlines

Stroke 'more likely' with long hours. BBC News, August 20 2015

Working longer hours increases stroke risk, major study finds. The Guardian, August 20 2015

Hard work really can kill, as longer hours increase risk of stroke. The Daily Telegraph, August 20 2015

Working long hours can increase stroke risk, research finds. Daily Mail, August 20 2015

Long hours at work increase risk of a stroke, scientists find. Daily Mirror, August 20 2015

Working long hours increase stroke risk, research says. ITV News, August 20 2015

Links to Science

Kivimäki P, Jokela M, Nyberg ST, et al. Long working hours and risk of coronary heart disease and stroke: a systematic review and meta-analysis of published and unpublished data for 603?838 individuals. The Lancet. Published online August 19 2015

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