Spending long hours at work can substantially increase the risk of stroke.
The research also reveals that those who work long hours may also be at higher risk of developing coronary heart disease.
The study, published in The Lancet, is the largest of its kind to investigate the link between working hours and cardiovascular health.
According to the Centers for Disease Control and Prevention (CDC), coronary heart disease and stroke are currently ranked first and fifth, respectively, in the top 10 causes of death in the US.
This is not the first time the effects of longer working hours have been examined. Last year, Medical News Today reported how individuals who were working between 61-70 hours a week had a 42% higher risk of developing coronary heart disease, compared with those working 40 hours or less.
According to a survey conducted last year, the average weekly working hours in the US exceeded the traditional 40 hours that are normally quoted by the government. Instead, full-time employees reported an average of 47 hours of work per week, which is almost an extra day in a standard Monday-Friday, 9-5 schedule.
To investigate, researchers from University College London (UCL) in the UK first conducted a systematic review and meta-analysis of studies from Europe, Australia and the US.
Stroke risk increases with working hours
Data from 25 studies was analyzed, involving more than 600,000 men and women from the three countries who were followed up for an average of 8.5 years. Researchers found individuals working 55 hours or more were at 13% higher risk for developing coronary heart disease.
This figure remained the same even after researchers took other risk factors into consideration, such as age, sex and socioeconomic status.
The risk of stroke was also found to be much higher for overworked individuals. The researchers analyzed data from 17 studies involving 500,000 men and women who were followed up for an average of 7.2 years. Results revealed the risk of stroke was 1.3 times, or 33%, higher for individuals who worked 55 hours or more per week.
Interestingly, researchers discovered the risk of stroke rose in conjunction with the amount of hours worked. Individuals working between 41-48 hours a week had a 10% higher of stroke, while for those working between 49-54 hours a week, the increased risk of stroke was almost three times higher at 27%.
Again, the increased risk remained even after accounting for other factors, such as smoking, alcohol consumption and physical activity. Standard cardiovascular risk factors including high blood pressure and high cholesterol were also taken into consideration, but the increased risk remained.
A 'pioneering' study
Prof. Mika Kivimäki, from UCL, believes the findings to be significant:
"The pooling of all available studies on this topic allowed us to investigate the association between working hours and cardiovascular disease risk with greater precision than has previously been possible. Health professionals should be aware that working long hours is associated with a significantly increased risk of stroke, and perhaps also coronary heart disease."
In a comment piece accompanying the study, Dr. Urban Janlert, from Umeå University in Sweden, describes the study as "pioneering." He adds:
"So far, Kivimäki and colleagues' results provide the strongest indication of a causal association between long working hours and an aspect of cardiovascular disease, namely, stroke."
Prof. Kivimäki hopes the new findings will help health care professionals take a more active role when advising patients who are overworking. He told MNT:
"Health care professionals should advise patients who work long hours that such working pattern is associated with an increased health risk and that management of vascular risk factors is particularly important for them, that is: keeping blood pressure, lipid levels and blood glucose within the normal range, adequate physical activity, eating and drinking healthfully, avoiding overweight and avoiding excessive stress."
Due to the substantial link between longer working hours and the increased risk of stroke, Prof. Kivimäki told MNT his aim is now "to better understand the mechanisms underlying the association between long working hours and stroke."