The new study "provides a firm anchor to state that shift work is associated with an increased risk of heart attack and stroke. The relationship is probably causal, but it is difficult to say that on the basis of observational studies alone," says study researcher Daniel G. Hackam, MD, PhD. He is an assistant professor of medicine at the University of Western Ontario, Canada.
The study is published online in the journal BMJ.
Exactly how shift work increases the risk for heart attack and stroke is not fully understood. It may disrupt the body's natural sleep-wake cycle. Shift workers may also be more likely to smoke and eat an unhealthy diet. And they may be less likely to get regular physical activity.
In the study, shift work was defined as:
any non-daytime schedule
Night shift workers in the study had the highest risk for heart attack and stroke, particularly in the first 10 to 15 years on the job.