Switching from day to night shift work for more than a few years was associated with a small increased risk for heart disease among women followed for more than 2 decades, researchers reported.
Women in the participating in the Nurses Health Study (NHS) who routinely switched from day or evening to night shifts for a decade or more had a 15% to 18% increased risk, compared with women who did not work rotating night shifts, according to Celine Vetter, PhD, of Brigham and Women's Hospital and Harvard Medical School in Boston, and colleagues.
Also, longer duration of rotating night shift work and recent rotating night shift work were associated with greatest risk., they wrote in the Journal of the American Medical Association.
But coronary heart disease (CHD) risk waned over time following cessation of rotating night shift work, so that after 2 decades of follow-up, the CHD risk associated with 10 years or more of rotating night-shift work was not significantly elevated, the authors noted.
Even though the absolute risk was small, and the contribution of shift work to CHD was modest, after adjustment for other heart disease risk factors, Vetter said it is significant that shift work is a modifiable risk factor for heart disease.
"We know the most important things women can do to lower their heart disease risk is maintain a healthy weight, eat a good diet, remain physically active, and refrain from smoking," she told MedPage Today. "But rotating shift work is also a modifiable risk factor. We need a better understanding of the connections between work schedules, individual characteristics, and coronary heart disease risk."
The analysis included around 189,000 participants in NHS I, which enrolled women with an average age of 54.5, and NHS II that enrolled younger nurses (average age 34.8).
NHS I participants were asked about their rotating night shift work history once in 1988, shortly after enrollment, and NHS II participants were asked every few years, through questionnaires, about their work schedules.
In the NHS I cohort (n=73,623), which followed women who were close to their end of their careers at enrollment, there were 7,303 incident cases of CHD identified during the 24-year follow-up.
Women who worked more than 10 years of rotating night shift work had an 18% increased risk for CHD versus women who did not work rotating shift work.
Their risk was more than twice as high during the first 12 years of follow-up (27%) compared with the second 12 years (13%), which strongly suggested waning risk over time following rotating night shift work cessation, Vetter said.
"When we looked at 4 year chunks of time, we saw that at the end of the 24 years there was really no association anymore," she said.
The NHS II cohort (n=115,535), which included women closer to the start of their careers, were first asked about their rotating shift work status in 1989. They were also asked to update this information every few years during follow-up. There were 3,519 incident CHD cases identified in this group.
Compared with women without a history of rotating night shift work, women who worked less than 5 years of shift work at baseline did not have a significantly increased CHD risk in age-adjusted analyses. (incidence rates, 425.5 and 122.6 per 100,000 person-years in the NHS I and NHS II, respectively),
Longer duration shift work was associated with an increased CHD risk, and the authors found the following:
- In NHS I, the incidence rate per 100,000 person years for 5-9 years of shift work was 525.7 (hazard ratio 1.21, 95% CI 1.11-1.33)
- In NHS I, the incidence rate per 100,000 person years for ≥10 years of shift work was 596.9 (HR 1.36, 95% CI 1.27-1.46, P<0.001 for trend)
- In NHS II, incidence rate for 5-9 years of shift work was 151.6 (HR 1.22, 95% CI 1.08- 1.38)
- In NHS II, incidence rate for ≥10 years of shift work was 178.0 (HR 1.34, 95% CI, 1.17-1.53, P<0.001 for trend)
"In this [NHS II] younger cohort,when using cumulatively updated shift work history, a higher CHD risk was observed with 12%, 19% and 27% increased risk for less than five year, five to nine years and 10 years or more of shift work, respectively," the researchers wrote. "Results were similar overall when restricting to women without hypertension, diabetes or hypercholesterolemia, suggesting that these conditions may not be the prime mediators of observed association between shift work and CHD."
Study limitations included the exclusion of men from the analysis, which limited generalizability, and the observational design, as well as the reliance on self-reports of shift work history and CHD status.
"Future studies should include a more detailed assessment of work hours and job demands, ideally in conjunction with chronotype and sleep timing measures, to enable more detailed studies of circadian strain on coronary health," the authors wrote.
This study was funded by the CDC and the National Institute for Occupational Safety.
Vetter disclosed support from the German Research Foundation.
Vetter and co-authors disclosed no relevant relationships with industry.
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