Night Shifts Upset Sleep, Up Heart Death Risk

Night shift work not only disrupts sleep patterns but also increases the risk of cardiovascular disease (CVD) and lung cancer mortality, according to one of the largest prospective cohort studies worldwide with a high proportion of rotating night shift workers and long follow-up time.

“Women working rotating night shifts for more than 5 years have a modest increase in all-cause and CVD mortality. Those working more than 15 years of rotating night shift work have a modest increase in lung cancer mortality. These results add to prior evidence of a potentially detrimental effect of rotating night shift work on health and longevity,” report researchers led by Eva S. Schernhammer, MD, DrPH, Associate Professor of Medicine, Harvard Medical School, and Associate Epidemiologist, Department of Medicine, Brigham and Women’s Hospital, Boston.

There is substantial biological evidence that night shift work enhances the development of CVD. In 2007, the World Health Organization classified night shift work as a probable carcinogen because of circadian disruption.

Sleep and the circadian system play an important role in cardiovascular health and antitumor activity. The circadian system and its prime marker, melatonin, are considered to have antitumor effects through multiple pathways-including antioxidant activity, anti-inflammatory effects, and immune enhancement-and they exhibit beneficial actions on cardiovascular health by enhancing endothelial function, maintaining metabolic homeostasis, and reducing inflammation, the researchers noted.

“Direct nocturnal light exposure suppresses melatonin production and resets the timing of the circadian clock,” they stated. “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, CVD, and ultimately, mortality.”

Using data from the Nurses’ Health Study, the international team of researchers analyzed 22 years of follow-up of nearly 75,000 women. Night shift information was collected in 1988. Rotating shift work was defined as working at least 3 nights per month in addition to days or evenings in that month.

The investigators found that working rotating night shifts for more than 5 years is associated with an increase in all-cause and CVD mortality. Mortality from all causes appeared to be 11% higher for women with 6 to 14 years or more than 15 years of rotating night shift work. CVD mortality appeared to be 19% and 23% higher for those groups, respectively. There was no association between rotating shift work and any cancer mortality, except for lung cancer in those who worked the night shift for 15 or more years (25% higher risk).

“A single occupation (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," the researchers noted.

“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 (eg, chronotype) warrant further exploration," they added.

The researchers published their results in the January 5, 2014 issue of the American Journal of Preventive Medicine.