Dr Martin Juneau, M.D., FRCP

Cardiologue, directeur de l'Observatoire de la prévention de l'Institut de Cardiologie de Montréal. Professeur titulaire de clinique, Faculté de médecine de l'Université de Montréal. / Cardiologist and Director of Prevention Watch, Montreal Heart Institute. Clinical Professor, Faculty of Medicine, University of Montreal.

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6 May 2019
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Overwork can increase the risk of cardiovascular disease

In Canada, as in most industrialized countries, the proportion of time spent working has decreased considerably since the beginning of the 20th century. At that time, most people worked around 3,000 hours per year (equivalent to 60–70 hours per week), much more than the 1,800 hours worked on average by today’s workers (Figure 1). This reduction in workload was particularly pronounced in Europe, with a 50% drop in hours worked between 1870 and 2000 (and even 60% in some countries such as Germany or Holland). Among the factors responsible for this trend is the importance of the workers’ movements, whose demands made it possible to limit the excesses and abuses that were frequent at the beginning of the Industrial Revolution (10 to 16 hours of work per day, 6 days a week) and to reduce the work week to a maximum of 40 hours.

Figure 1. Decrease in the number of hours worked annually between 1870 and 2000 in Canada (red) and Europe (black). Adapted from Huberman and Minns (2007).

This reduction in workload, however, does not affect all workers: in some countries, there is a significant proportion of people who still work a lot (more than 50 hours per week), a situation that affects more than one in five workers in Turkey, Mexico, Japan and Korea (Figure 2). In Western countries (United Kingdom and the United States) as well as in Australia, long hours at work are also quite frequent (about 15% of workers), while this phenomenon is much less common in Canada as well as in European countries.

Figure 2. Comparison of the proportion of employees who worked more than 50 hours per week in 2016 in different OECD (Organization for Economic Co-operation and Development) countries. From OECD.Stat.

It should also be noted that since the 1990s, deindustrialization and the technological revolution that have affected economically developed countries have fostered the emergence of a new economy, mainly based on services. There are several consequences to this transformation, one of them being that it has become commonplace to work more than 8 hours a day to meet the demands imposed by work. This type of situation is particularly common among high-skilled workers (finance, health, technology, etc.), with more than one in five people routinely working more than 50 hours a week (Figure 3). Lower skilled workers earning lower wages are not spared, however, since many of them have to combine part-time jobs to make ends meet. In other words, even though the majority of people are currently working less than before, there are still two sub-populations of hard workers: highly skilled workers who have to work long hours in exchange for high wages and/or to keep their jobs, and low-skilled, low-wage workers who have to work longer because of their more precarious financial situation.

Figure 3. Increase in the proportion of people who work a lot (50 hours a week or more) among highly skilled workers in Western Europe and North America. From Burger (2015).

Effects on cardiovascular health
An immediate consequence of the long hours spent at work is increasing stress levels, a major risk factor for cardiovascular disease. One of the first clues to this comes from the INTERHEART study, where work-related stress was associated with a twice as high risk of coronary heart disease, an increase similar to that observed for well-documented stressors such as divorce or the death of a loved one. This detrimental effect of overwork on health is particularly well documented in Japan. Loyalty, a sense of duty and sacrifice, and respect for the hierarchy occupy a very important place in Japanese culture; as a result, the number of hours spent at work is considered proof of loyalty to the company, and it is therefore very common to work a lot (up to 60 hours or more per week) to please superiors and keep the respect of colleagues. This extreme “dedication” to the company particularly increased in the 1990s because of what has been called the “Lost Decade”, characterized by economic stagnation, lower wages, and greater job insecurity.

During this period, there was a significant increase in mortality caused by overwork, a phenomenon known in Japan as karōshi (karō “overwork” and shi “death”). In most cases, these premature deaths are a consequence of the detrimental influence of long hours worked on the cardiovascular system: for example, an analysis of the causes of death of 203 karōshi victims revealed that 60% had died of a stroke (meningeal or intracerebral haemorrhage and cerebral infarction), 25% had acute heart failure, 13% had a heart attack, and 2% had an aortic rupture. In addition to these sudden cardiac deaths, it should be noted that overwork is also an important cause of suicide (karōjisatsu), which can account for up to 12% of voluntary deaths in Japan. Similar effects of overwork were observed in other Asian countries, including South Korea (gwarosa) and China (guolaosi); in the latter case, it has become common for Chinese companies to introduce what is familiarly known as “996”, that is, a work schedule from 9 a.m. to 9 p.m., 6 days a week.

In Western countries, a meta-analysis of studies of 600,000 workers in Europe, Australia and the United States showed a strong association between long hours worked and the risk of cardiovascular disease. This analysis revealed that people who work a lot (more than 55 hours per week) are about 13% more likely to have a heart attack than those who work 35–40 hours a week, while the risk of stroke was increased by 33%. This increased risk of stroke, already observed among people who work 41–48 hours (10%), increases to 27% among those who work 49–54 hours, and finally reaches 33% among very hard workers (55 hours and more).

Several psychosocial, behavioural and biological factors have been proposed to explain how stress may contribute to the increased risk of cardiovascular events seen in hard workers (Table 1). However, it should be noted that a link between excessive work and these different factors has not been observed in all studies and their exact contribution remains to be clearly established.

Table 1. Major factors that may contribute to the increased risk of cardiovascular disease caused by overwork.

Risk behaviours (smoking, excess alcohol)People who work more than 50 hours per week are more likely to smoke (Artazcoz et al. 2009) and drink excessive amounts of alcohol (Virtanen et al. 2015).
Lack of sleepWorking more than 55 hours per week is associated with an increased risk of not getting enough sleep (twice), having difficulty falling asleep (4 times), and fatigue when waking up (twice). (Virtanen et al. 2009).
HypertensionPeople who work more than 50 hours per week are 30% more likely to have high blood pressure (Yang et al. 2006).
Sedentary lifestyleThe high number of hours spent at work reduces the time available for other activities, including exercise. Studies show that physical inactivity, combined with sedentary work, increases the risk of cardiovascular disease (Ekelund et al. 2016).
Fatigue and exhaustionYoung men (<55 years) who had an acute myocardial infarction frequently report episodes of excessive fatigue and exhaustion in the period prior to the coronary heart event (Sihm et al. 1991).
Job strainJob strain, a situation where the worker faces high demands but lacks the resources to meet them, is associated with a 23% increase in the risk of coronary heart disease (Kivimäki et al. 2012) and a 30% increase in the risk of stroke (Huang et al. 2015)
Atrial fibrillationPeople who work long hours are at higher risk for episodes of atrial fibrillation, a major risk factor for stroke (Kivimäki et al. (2017).

The increased risk of atrial fibrillation caused by long working hours is particularly interesting because this arrhythmia causes clots in the left atrium, which can reach the brain and obstruct blood flow to the brain, and could contribute to the increased risk of stroke in people who work hard. This increase in the incidence of atrial fibrillation is mainly observed in people who work more than 50 hours a week, and can reach up to 40% of those who work more than 55 hours a week (Figure 4). It should also be noted that overwork has been associated with other coagulation disorders, including deep vein thrombosis (a 68% increase risk), a major cause of pulmonary embolism.

Figure 4. Effect of the number of hours worked on the risk of atrial fibrillation.
From Kivimäki et al. (2017).

Job strain
Another factor that may contribute to the negative effects of overwork is the so-called job strain, a particular form of stress that has been repeatedly associated with cardiovascular health risk situations. According to the model developed by Robert Karasek (see box), this tension at work is defined as a situation where work and psychological demands are high, combined with a lack of available resources to cope with it and a low decision-making latitude. In other words, a situation where workers are required to be highly productive, but without giving them adequate resources or the necessary flexibility. The risk is further aggravated if the employee has low social support (e.g. from co-workers) and/or if the high productivity requirement is not associated with a valuation of the work done (effort-reward imbalance). It goes without saying that working long hours in such an unfavourable environment can only have a negative impact on both physical and mental health.

Job strain
  A work situation is always the result of a combination of two factors: 1) A “psychological demand”, that is to say, the demands imposed by the job (amount of work to be done, time constraints, interruptions, contradictory demands, etc.); 2) “job decision latitude”, i.e., the ability to make decisions and be creative, for example by having the opportunity to choose how to do one’s job, to participate in decisions, and to use skills. According to Karasek, the combination of psychological demand and decision latitude makes it possible to define 4 types of work situations (see figure below)

The at-risk combination is one in which the psychological demand is high, either because of the amount of work to be done or its difficulty, combined with the depreciation of the workers due to their low participation in the decision-making process.

In conclusion, overwork is associated with a slight but significant increase in the risk of cardiovascular disease, especially stroke. It should be noted, however, that recent studies suggest that the effect of psychological stress on cardiovascular health is more pronounced in people who already have cardiometabolic abnormalities. For example, it has been shown that the association between work strain and risk of premature mortality was higher among those with a history of diabetes, coronary heart disease, or stroke than among those who had not been affected by these conditions. These at-risk individuals generally have several metabolic abnormalities (high blood sugar, inflammation, atherosclerotic plaques) that increase the risk of cardiovascular events following exposure to the effects of work stress (arrhythmia, hypercoagulation, hypertension, etc.). Conversely, people who work hard but who exercise regularly and are in good physical shape do not show any increase in the risk of coronary heart disease. As with the general population, it is therefore important for hard workers to adopt a generally healthy lifestyle (normal weight, plant-rich diet, regular physical activity) to reduce the effects of stress on the risk of cardiovascular events. Working has never killed anyone … who is in good health.

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