A new study has found that people who work night shifts are at increased risk of developing atrial fibrillation (AF), an irregular and often abnormally fast heart rhythm.

The study, which was published in the European Heart Journal, is the first to investigate the link between AF and night shift work.

The researchers used information from more than 283,000 people in the UK Biobank database, and found that the longer and more frequently that people worked night shifts over their lifetimes, the greater their risk of AF. Working nights was also linked to an increased risk of heart disease, but not stroke or heart failure.

The study also investigated whether genetic predisposition to AF could play a role in the increased risk. They evaluated the overall genetic risk on the basis of 166 genetic variations known to be associated with the condition, but found that the genetic risk levels did not affect the link between working night shifts and AF risk, regardless of whether participants had a low, medium or high genetic risk.

The researchers adjusted their analyses for factors that could affect the results, such as age, sex, ethnicity, education, socioeconomic status, smoking, physical exercise, diet, body mass index, blood pressure, sleep duration and chronotype (whether someone was a ‘morning’ or an ‘evening’ person).

Women were more susceptible to AF than men when working night shifts

They found that people who currently worked night shifts on a usual or permanent basis had a 12% increased risk of AF compared to people who only worked during the day. The risk increased to 18% after ten or more years for those who had a lifetime duration of night shifts. For people who worked an average of three to eight night shifts a month for ten years or more, the risk of AF increased to 22% compared to daytime workers.

Professor Lu Qi, of Tulane University School of Public Health and Tropical Medicine, New Orleans, USA, co-led the study. She said: “There were two more interesting findings. We found that women were more susceptible to atrial fibrillation than men when working night shifts for more than ten years. Their risk increased significantly by 64% compared to day workers.

“People reporting an ideal amount of physical activity of 150 minutes a week or more of moderate intensity, 75 minutes a week or more of vigorous intensity, or an equivalent combination, had a lower risk of atrial fibrillation than those with non-ideal physical activity when exposed to a lifetime of night shift work. Thus, women and less physically active people may benefit particularly from a reduction in night shift work.”

Reducing the frequency and the duration of night shift work may be beneficial

While the study has many strengths, including its size and detail, there are some limitations. The study cannot prove that shift work causes heart problems, only that they are associated. The authors also highlight that some cases of atrial fibrillation may have been missed; lifetime employment was assessed only when people joined UK Biobank, was self-reported and, therefore, may have changed or be prone to some errors; there may be unknown factors that might affect the results; and the people in UK Biobank were mainly white British and so it may not be possible to generalise the findings to other ethnic groups.

Professor Yingli Lu, co-author of the study, said: “Although a study like this cannot show a causal link between night shifts and atrial fibrillation and heart disease, our results suggest that current and lifetime night shift work may increase the risk of these conditions.

“Our findings have public health implications for preventing atrial fibrillation. They suggest that reducing both the frequency and the duration of night shift work may be beneficial for the health of the heart and blood vessels.”