Another Study Linking Night Shifts to Increased CHD Offers Insights Into Dose-Response

Yet another study is demonstrating a powerful link between many years of rotating night shifts and a greater risk of developing coronary heart disease.  

The latest data come from a large cohort of female registered nurses and show that those who reported working three or more night shifts per month in addition to day and evening shifts for at least 5 years had a 12% to 18% elevated risk of CHD events compared with nurses who did not report such work. The risk grew with increasing time spent working rotating night shifts.

But in a new finding that hints at some of the directions future research could take, the study also demonstrates a decrease in risk with a greater time since quitting such work.

“This is a novel finding that hasn’t been described before and warrants replication,” said lead author Céline Vetter, PhD (Brigham and Women’s Hospital, Boston, MA), in an interview with TCTMD.

Dose-Response Relationship

The study, published in the April 26, 2016, issue of the Journal of the American Medical Association, looked at incident CHD outcomes in 189,158 initially healthy registered nurses who were followed for over 24 years in the ongoing, prospective Nurses’ Health Study I and II (NHS I and NHS II). Participants completed questionnaires that included a shift work assessment (lifetime assessment in the NHS I and ongoing, updated assessments in the NHS II) and were asked to report physician-diagnosed CHD events.

A total of 10,822 CHD events were reported over the study period. Women who worked less than 5 years of shift work, which comprised the largest group of women in both the NHS I and NHS II, had no increased risk of CHD compared with non-shift workers. But those in both studies who reported 5 to 9 years of such work had a 12% increased incidence. The risk for events was even greater among those with 10 or more years of shift work exposure (HR 1.18 compared with baseline in the NHS I and HR 1.15 compared with baseline in the NHS II). Vetter and colleagues say the findings suggest that CHD risk associated with shift work changes over time.

The association between length of shift work and CHD risk remained strong even after adjusting for hypertension, diabetes, and high cholesterol. In an analysis stratified by body mass index, women who were obese and worked rotating night shifts had higher risk of CHD compared with normal-weight or overweight women.

The link between lifetime years of shift work and CHD was stronger in the first 12 years of follow-up in the NHS I than in the second 12 years (P = 0.02 for interaction), raising the possibility of a health benefit of cessation. Additionally, longer time since quitting shift work was associated with decreased CHD risk in the NHS II (P < 0.001 for trend).

Potential for Changes to Benefit Workers

Several prior studies have also shown an association between night shift work and increased risk for cancer, stroke, and other health problems, including a 2012 meta-analysis showing that shift work increased the risk of any coronary event by about 24%.

In an email, Daniel G. Hackam, MD, PhD (University of Western Ontario, London, Canada), senior author of that meta-analysis, said the current study “adds to the literature, suggesting adverse vascular health consequences, especially from night shift work.”

But one of the problems with this type of research, Vetter noted, is that many of the studies have been inconsistent in their definition of shift work. She said her study adds to the body of research in this field by attempting to be more specific about frequency and duration of shift work, as well as by using a broader endpoint definition of CHD that included nonfatal MI, CHD death, angiogram-confirmed angina, any CABG, stenting, or angioplasty.

“We know that coronary heart disease is a long process and there are a lot of procedures that have been developed over time to treat [it],” Vetter said. “If you don’t take these into account, effects could be concealed by secondary or tertiary prevention.”

She added that the Nurses’ Health Study datasets offer a unique opportunity to look closely at this topic but said that more specific information that is not collected, such as work start and end times, consecutive days worked, and frequency of rotations, could make a difference in understanding the association and finding ways to make changes to decrease health risk.

Taken together, she said the findings indicate “a small and modest contribution of shift work to coronary heart disease. But it’s interesting because it’s potentially modifiable.” While the mechanisms behind the association are unclear, the effect of circadian rhythm disturbances on downstream physiology is the most likely culprit, she said, adding that ongoing circadian studies may provide important information that could decrease the misalignment of biological hours and working hours that occurs in those who work shifts.

“Understanding the threshold of the strain to the system is going to be the challenge for the near future,” Vetter said. “Right now we don’t really know who is at highest risk, and we also don’t really know which aspects of work schedules might carry a higher risk for some people than for others.”

Disclosures:

• Vetter and Hackam report no relevant conflicts of interest. 

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