Study finds one week of night shifts per month does not increase the risk of coronary heart disease

If you clock in for work in the early hours when most of the population is still asleep, you may have heard that it increases the risk of heart disease. But now there is good news from Aarhus University for those of us who work night shifts. A new study shows no increased risk of coronary heart disease if you work up to seven night shifts per month.

Ph.D. Student Jesper Medom Vestergaard from the Department of Clinical Medicine is responsible for the study, which is based on working time registrations of more than 250,000 employees from 2007 to 2015, combined with their hospital records. The results are published in the International Journal of Epidemiology.

"In our study, we didn't see any association between the number of night shifts and the risk of coronary heart disease. Many international studies have otherwise suggested an association, which is why our results are interesting, not least because the previous studies are primarily based on questionnaires, whereas we've had an opportunity to compare specific data on working hours with people's health information," says Jesper Medom Vestergaard.

In the study, Jesper Medom Vestergaard had access to day-to-day information on working hours for a total of 254,031 health care workers and other employees in the Danish regions, and this information was linked with their health information, including information from the Danish National Patient Register.

"Employees in the regions work on average 1.8 night shifts per month, and 93% of them work fewer than seven monthly night shifts. The study shows that this is not associated with an increased risk of coronary heart disease."

Jesper Medom Vestergaard emphasizes, however, that the results of the study do not mean that everyone working night shifts can breathe a sigh of relief.

"Most of the 250,000 participants in our study work in the health care sector, and few of them work nights permanently. Therefore, we can't conclude that there is not an increased risk of coronary heart disease associated with night shifts. Many other factors may be in play, but the data in the analyses were adjusted for age, sex, family history of cardiovascular disease and other diseases associated with coronary heart disease."

Jesper Medom Vestergaard will follow up with a new study, in which he will look at the risk of breast cancer, which has also previously been linked to night work.