Overtime 'leads to heart risks'

Working just three hours of overtime each day will raise the risk of heart disease by 60%, according to the Daily Mail.

The news comes from a study that compared the working habits of 6,000 civil servants to their risk of heart disease over an 11-year period. Even after discounting for the influence of cardiovascular risk factors, such as smoking, there was still a 60% greater risk of developing heart disease among people who worked three to four hours of overtime each day. Working one or two extra hours did not seem to have any effect.

There were some small problems with this research, including the possibility of chance findings due to the small number of people who developed heart disease in the high overtime group. However, this was a well-conducted study, and overall it suggests that working three to four hours of overtime every day may increase the risk of heart disease. It is not yet clear why this was the case, therefore further research will be needed to identify the mechanisms behind the relationship.

Where did the story come from?

The study was carried out by researchers from the Finnish Institute of Occupational Health, University College London, and other institutions in France and Turkey. It was funded by grants from various sources, including the Medical Research Council and British Heart Foundation. The study was published in the European Heart Journal, a peer-reviewed medical journal.

The news stories have generally reflected the findings of this research accurately.

What kind of research was this?

This research was part of the Whitehall II Study, a large cohort study on members of the civil service, which has provided data for numerous other pieces of medical research. The study has been ongoing for 25 years, but this particular part of the research followed participants for an average of 11 years to see how work characteristics related to the development of coronary heart disease (CHD). To help maintain accuracy of results, a study of this type would need to ensure that people were free of CHD at the study’s start. It would also need to make appropriate adjustments for other confounding factors when assessing the relationship between working habits and cardiovascular events.

What did the research involve?

The Whitehall II study commenced in 1985, recruiting British civil servants aged between 35 and 55. The study has conducted several follow-up phases: this new study is concerned with those participants who provided responses to questions on working hours between 1991 and 1994. After excluding those who did not work full-time, had not responded to the question on working hours, or already diagnosed with CHD, the study included 6,014 adults (4,262 men and 1,752 women) who were then aged 39–61 years. The participants were followed for an average of 11 further years, and were each given a clinical examination between 2002 and 2004.

Assessment of work at baseline (1991-94) included the question “On an average weekday, approximately how many hours do you spend on work (during the daytime and work brought home)?” Responses on total working hours ranged from 1-12 hours. They were grouped into overtime categories of no overtime (7–8 hour normal working day); one hour of overtime every day (9 hour day); two hours (10 hour day); or three to four hours of overtime (11–12 hour day).

Cases of CHD up to 2002-04 were assessed by flagging participants’ records in the NHS Central Registry, which would notify the researchers of any deaths and their causes. They also used the registry to identify non-fatal heart attacks, and clinical records to identify angina. In analysing the relationship between working hours and CHD, the researchers adjusted for a wide variety of social, demographic and work-related factors, such as job role, salary, absence due to sickness, medical factors, diet, exercise and sleep.

What were the basic results?

Out of the 6,014 participants, 3,256 (54%) did not usually work overtime, 1,247 (21%) worked approximately one extra hour, 894 (15%) worked two extra hours, and 617 (10%) worked three or four extra hours a day.

After an average of 11 years and 67,544 person-years of follow-up, there were a total 369 cases of fatal CHD, non-fatal heart attacks or definite angina. When adjusting for social and demographic characteristics, three to four hours of overtime per day increased the risk of any of these outcomes by 60% compared to employees who did no overtime (hazard ratio 1.60, 95% confidence interval [CI] 1.15 to 2.23).

The relationship remained significant after adjusting for 21 potential confounding risk factors, such as BMI, smoking and cholesterol (HR 1.56, 95% confidence interval [CI] 1.11 to 2.19). Working less than one or two extra hours per day was not associated with increased risk.

How did the researchers interpret the results?

The researchers concluded that working overtime is linked to an increased risk of developing coronary heart disease. This link was still evident even after discounting the effects of other known risk factors.

Conclusion

This cohort study examining the association between overtime work and the development of heart disease has a number of strengths. It examined a large cohort of more than 6,000 people, it carried out regular follow-ups and collected extensive data on a wide range of medical, lifestyle and socio-demographic factors. Furthermore, this large range of data was taken into account when examining the relationship between heart disease and overtime. The methods of detecting new cases of CHD during follow-up also seem to be reliable. However, there are some points to note:

• The study only found a significant relationship between working three to four extra hours per day. Although the overall study had a large sample, there were only 51 people who worked this amount of overtime and developed CHD during follow-up. Using such a small number of individuals to calculate risk increases the likelihood of producing chance findings.
• There was no relationship between usually working one or two extra hours and CHD. This result seems at odds with the relationship seen in those working longer hours.
• Overtime was only assessed at one time point. Although the question asked the participants how many hours they spent working on a typical workday, the responses may not reflect a regular pattern for everyone.
• Also, overtime may constitute different things to different people. This could in turn lead to different perceptions of stress and workload. In particular, these results in the civil service population may not apply to other professions.
• Although the research adjusted for many confounders, there is the possibility that others have not been examined. Importantly, mental health diagnoses such as depression or anxiety were not examined.
• There is the possibility that people with existing CHD may have had their illness missed at the time of the baseline questionnaire, meaning they would have been incorrectly included in the study. Equally, new cases of CHD may have been missed during follow-up.

On the basis of this study, it can only be concluded that working three to four hours of overtime everyday may increase the risk of heart disease. Further research is needed to identify how longer work hours affect the body in ways that increase the risk of coronary heart disease.