Importance Prospective studies linking shift work to coronary heart disease (CHD) have been inconsistent and limited by short follow-up.
Objective To determine whether rotating night shift work is associated with CHD risk.
Design, Setting, and Participants Prospective cohort study of 189 158 initially healthy women followed up over 24 years in the Nurses’ Health Studies (NHS [1988-2012]: N = 73 623 and NHS2 [1989-2013]: N = 115 535).
Exposures Lifetime history of rotating night shift work (≥3 night shifts per month in addition to day and evening shifts) at baseline (updated every 2 to 4 years in the NHS2).
Main Outcomes and Measures Incident CHD; ie, nonfatal myocardial infarction, CHD death, angiogram-confirmed angina pectoris, coronary artery bypass graft surgery, stents, and angioplasty.
Results During follow-up, 7303 incident CHD cases occurred in the NHS (mean age at baseline, 54.5 years) and 3519 cases in the NHS2 (mean age, 34.8 years). In multivariable-adjusted Cox proportional hazards models, increasing years of baseline rotating night shift work were associated with a significantly higher CHD risk in both cohorts. In the NHS, the age-standardized incidence rate (IR) for less than 5 years of rotating night shift work per 100 000 person-years was 435.1 (hazard ratio [HR], 1.02; 95% CI, 0.97-1.08); the IR for 5 to 9 years of rotating night shift work was 525.7 (HR, 1.12; 95% CI, 1.02-1.22); and the IR for 10 years or more of rotating night shift work was 596.9 (HR, 1.18; 95% CI, 1.10-1.26; P<.001 for trend) vs an IR of 425.5 in women who never worked rotating night shifts. In the NHS2, the IR for less than 5 years was 130.6 (HR, 1.05; 95% CI, 0.97-1.13); the IR for 5 to 9 years was 151.6 (HR,1.12; 95% CI, 0.99-1.26); and the IR for 10 years or more was 178.0 (HR, 1.15; 95% CI, 1.01-1.32; P = .01 for trend) vs an IR of 122.6 in women who never worked rotating night shifts. In the NHS, the association between duration of rotating night shift work and CHD was stronger in the first half of follow-up (IR for <5 years, 382.4; HR, 1.10 [95% CI, 1.01-1.21]; IR for 5-9 years, 483.1; HR, 1.19 [95% CI, 1.03-1.39]; and IR for ≥10 years, 494.4; HR, 1.27 [95% CI, 1.13-1.42]; P<.001 for trend) than in the second half (IR for <5 years, 424.8; HR, 0.98 [95% CI, 0.92-1.05]; IR for 5-9 years, 520.7; HR, 1.08 [95% CI, 0.96-1.21]; IR for ≥10 years, 556.2; HR, 1.13 [95% CI, 1.04-1.24]; P = .004 for trend; P = .02 for interaction), suggestive of waning risk after cessation of shift work. Longer time since quitting shift work was associated with decreased CHD risk among ever shift workers in the NHS2 (P<.001 for trend).
Conclusions and Relevance Among women who worked as registered nurses, longer duration of rotating night shift work was associated with a statistically significant but small absolute increase in CHD risk. Further research is needed to explore whether the association is related to specific work hours and individual characteristics.