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UCL study: Overtime ‘bad for your heart’

12 May 2010

Engraving of human heart

Working overtime is bad for the heart according to the results from a long-running UCL-led study of more than 10,000 civil servants in London.

The research, published online today in the European Heart Journal, found that, compared with people who did not work overtime, people who worked three or more hours longer than a normal, seven-hour day had a 60% higher risk of heart-related problems such as death due to heart disease, non-fatal heart attacks and angina.

Dr Marianna Virtanen, an epidemiologist at UCL and the Finnish Institute of Occupational Health, said: “The association between long hours and coronary heart disease was independent of a range of risk factors that we measured at the start of the study, such as smoking, being overweight, or having high cholesterol.

“Our findings suggest a link between working long hours and increased CHD [coronary heart disease] risk, but more research is needed before we can be confident that overtime work would cause CHD. In addition, we need more research on other health outcomes, such as depression and type 2 diabetes.”

The Whitehall II study started in 1985 and recruited 10,308 office staff aged 35 – 55 from 20 London-based civil service departments. Data has been collected at regular intervals and in the third phase, between 1991 and 1994, a question on working hours was introduced. The current analysis looked at the results from 6,014 people (4,262 men and 1,752 women), aged 39 – 61, who were followed from 2002 to 2004, the most recent phase for which clinical examination data is available.

During the average 11.2 years of follow-up, Dr Virtanen and her colleagues found that there had been 369 cases of fatal CHD, non-fatal heart attacks (myocardial infarctions) or angina. After adjusting for sociodemographic factors such as age, sex, marital status and occupational grade, they found that working three to four hours overtime (but not one to two hours) was associated with a 60% higher rate of CHD compared with no overtime work. Further adjustments for a total of 21 risk factors made little difference to these estimates.

The researchers said there could be a number of explanations for this association between overtime and heart disease. Their results showed that working overtime was related to type A behaviour pattern (people with type A behaviour tend to be aggressive, competitive, tense, time-conscious and generally hostile), psychological distress manifested by depression and anxiety, and possibly with not enough sleep, or not enough time to unwind before going to sleep.

Other possible explanations include: high blood pressure that is associated with work-related stress but is ‘hidden’ because it doesn’t necessarily show up during medical check-ups; ‘sickness presenteeism’ whereby employees who work overtime are more likely to work while ill, ignore symptoms of ill health and not seek medical help; and, finally, it is possible that people in jobs where they have more freedom or latitude over their work-related decisions may have a lower risk of CHD despite working overtime.

However, Dr Virtanen said that the findings were independent of all of these factors, and so they could not necessarily provide the full explanation for why overtime was associated with the higher risk of heart disease.

She added: “We did not measure whether subsequent changes in these factors during the follow-up period altered the association. One plausible explanation for the increased risk could be that adverse lifestyle or risk factor changes are more common among those who work excessive hours compared with those working normal hours. Another possibility is that the chronic experience of stress (often associated with working long hours) adversely affects metabolic processes. It is important that these hypotheses should be examined in detail in the future.”

The authors warned that the study did not include blue-collar workers, so it remained unclear whether the findings could be applied to blue-collar workers and employees in the private sector.

Senior author Mika Kivimäki, Professor of Social Epidemiology at UCL, said future research would involve analysing data over periods of time to examine whether working long hours predicts changes in lifestyle, mental health and traditional risk factors, such as blood pressure, blood glucose and cholesterol.

He added: "We hope that this research will increase understanding of the mechanisms underlying the association between long working hours and coronary heart disease. We will also examine whether overtime work increases the risk of depression, as recent research suggests that depression increases the risk of coronary heart disease.”

Image: An engraving of the human heart

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