Study: Data from nurses’ study finds link between night shifts, higher mortality risk

A new analysis of data from the world’s largest and longest-running study of women’s health finds that rotating night shift work is associated with higher mortality rates. The new findings add to a growing awareness that long-term night shift work comes with serious occupational health risks.

Published this month in the American Journal of Preventive Medicine, the study found that all-cause and cardiovascular disease-related mortality were significantly increased among women who worked more than five years of rotating night shifts when compared to those who never worked the night shift. In addition, the study found that working 15 or more years of rotating night shifts was associated with a modest increase in lung cancer mortality. Previous research has also found a link between working the night shift and serious health risks. In fact, in 2007, the World Health Organization designated night shift work as a probable carcinogen, as it disrupts the physical, mental and behavioral changes that follow a daily cycle — otherwise known as circadian rhythms. Study authors Fangyi Gu, Jiali Han, Francine Laden, An Pan, Neil Caporaso, Meir Stampfer, Ichiro Kawachi, Kathryn Rexrode, Walter Willett, Susan Hankinson, Frank Speizer and Eva Schernhammer write:

The circadian system and its prime marker, melatonin, are considered to have anti-tumor effects through multiple pathways, including antioxidant activity, anti-inflammatory effects, and immune enhancement. They also exhibit beneficial actions on cardiovascular health by enhancing endothelial function, maintaining metabolic homeostasis, and reducing inflammation. Direct nocturnal light exposure suppresses melatonin production and resets the timing of the circadian clock. In addition, sleep disruption may also accentuate the negative effects of night work on health. Taken together, substantial biological evidence supports the role of night shift work in the development of poor health conditions, including cancer, (cardiovascular disease), and ultimately, mortality.

To conduct the study, researchers analyzed data from the Nurses’ Health Study, which was established in 1976 and involved nearly 122,000 nurses. The night shift analysis was based on 22 years of health and behavioral data follow-up among nearly 75,000 of the participating nurses. In a press release about the findings, study co-author Schernhammer, an associate professor at Harvard Medical School and associate epidemiologist at Brigham and Women’s Hospital, described the study as “one of the largest prospective cohort studies worldwide with a high proportion of rotating night shift workers and long follow-up time. A single occupation (in this case, nursing) provides more internal validity than a range of different occupational groups, where the association between shift work and disease outcomes could be confounded by occupational differences.”

In analyzing the decades of data, Schernhammer and her colleagues found that all-cause mortality appeared to be 11 percent higher for nurses who worked six to 14 years of rotating night shifts, which was defined as working nights at least three times per month. In addition, cardiovascular disease-related mortality appeared to be 19 percent higher for those working rotating night shifts for six to 14 years as well as 23 percent higher for those working such shifts for 15 years or more. No association was found between rotating night shifts and cancer mortality, except in the case of lung cancer — nurses who worked rotating night shifts for 15 years or more appeared to experience a 25 percent higher risk of lung cancer mortality. The researchers noted that while the impact of rotating night shifts appeared to be stronger among current smokers in regard to all-cause mortality, the effect of night shift work was still “statistically significant” among nurses who never smoked.

The researchers noted that while the study helped confirm previous findings on the adverse effects of night shift work, more research is needed to inform and shape feasible interventions for such workers.

“To derive practical implications for shift workers and their health, the role of duration and intensity of rotating night shift work and the interplay of shift schedules with individual traits…warrant further exploration,” the study concluded.

To read a full copy of the study, visit the American Journal of Preventive Medicine.

Kim Krisberg is a freelance public health writer living in Austin, Texas, and has been writing about public health for more than a decade.

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