For quite some time, it's been believed that working long hours can lead to health problems, especially an increased risk of heart attack and stroke, and a new study from University College, London, backs up the belief.

The study, which is published in the journal Lancet, found that working 55 hours or more a week increases the risk of stroke by 33 percent when compared to working a standard 35 to 40 hour week.

To study stroke, the researchers analyzed 17 studies including 528,909 men and women who were followed for an average of 7.2 years, and found the risk of stroke was 1.3 times higher in those who worked 55 hours a week or more. The link remained even after taking into account behaviors such as smoking, alcohol consumption, and physical activity.

To study heart attacks, researchers analyzed 25 published and unpublished studies which examined the effects of long working hours on cardiovascular disease. The studies included 603,838 men and women from Europe, Australia, and the United States who were followed an average of 8.5 years. Longer working hours were associated with a 13 percent increased risk of heart disease (a new diagnosis, hospitalization, or death), even after considering risk factors such as age, sex, and socioeconomic status.

"Sudden death from overwork is often caused by stroke and is believed to result from a repetitive triggering of the stress response," the researchers wrote. “Behavioral mechanisms, such as physical inactivity, might also link long working hours and stroke; a hypothesis supported by evidence of an increased risk of incident stroke in individuals who sit for long periods at work."

"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," said Mika Kivimäki, Professor of Epidemiology at University College London. "Health professionals should be aware that working long hours is associated with a significantly increased risk of stroke, and perhaps also coronary heart disease."

Heart disease is the leading cause of death in the United States and accounts for about 1 in 3 deaths, according to the American Heart Association.