BOSTON, MA — Although past studies have linked night-shift work and increased coronary heart disease (CHD) risk, a prospective cohort study suggests the story isn't so simple.
"This is really the first report where we see that the risks associated with shift work seem to wane over time when you stop working shifts, and this is still adjusting for significant and important covariates such as changes in [body-mass index] BMI, physical activity, diet, and smoking," principal investigator Dr Céline Vetter (Channing Division of Network Medicine, Harvard University, Boston, MA) told heartwire from Medscape.
The analysis of 189,158 women over 24 years in the Nurses' Health Study (NHS) and NHS2 showed increased risks of 18% and 27%, respectively, for incident CHD for those who worked 10 years or more of rotating night-shift work, compared with those with no such history.
The association was significantly stronger in the first vs the second half of follow-up in the NHS, when women were less likely to work rotating night shifts (27% vs 13%; P=0.02).
In NHS2, which was a younger cohort with more frequent shift-work assessments, CHD risk was increased 38% in current night-shift workers and 25%, 13%, and 0.97%, respectively, in those with <12 years, 12 to 24 years, and >25 years since quitting night-shift work, the investigators reported in the April 26, 2016 issue of the Journal of the American Medical Association.
More than 15 million Americans work overnight, and up to 25% of workers worldwide are estimated to have such schedules. The authors say the analysis supports the hypothesis that the disruption of the biological and social rhythms associated with shift work could be driving increased CHD risk.
"If you suffer from circadian misalignment we know that there is increased inflammation, we know that metabolism is dysregulated. So most of those mechanisms that we think of that are connected with coronary artery disease are basically linked to the circadian system," Vetter said.
She described the overall contribution of shift work to CHD as "modest" but said it's important because it's potentially modifiable. Recent research has shown, for example, that taking factory night shifts away from "morning larks" and morning shifts from "night owls" can significantly increase sleep duration and decrease circadian misalignment.
"We have some ideas about where to go, but we don't have enough evidence right now to tell people what to change or how employers should design schedules," Vetter said.
The investigators defined rotating night-shift work as three or more night shifts per month in addition to day and evening shifts at baseline. Participants in NHS (mean age 54.5 years) were asked about shift work once and every 2 to 4 years in NHS2 (mean 34.8 years).
Multivariable analyses demonstrated a significant dose-response relationship between rotating night-shift work exposure and the CHD outcome of CABG surgery, angiogram-confirmed angina pectoris, angioplasty, stents, MI, and CHD death, which is broader than used in most prior studies.
Baseline History of Rotating Night-Shift Work
Group None 5 y 5–9 y >10 y P for trend
CHD incidence ratea 425.5 435.1 525.7 596.9
HRb 1 1.02 1.12 1.18 <0.001
CHD incidence ratea 122.6 130.6 151.6 178.0
HRb 1 1.05 1.12 1.15 0.01
a. Age-adjusted rates per 100,000 person-years
b. Multivariable adjusted hazard ratio
The authors note that the overall CHD risk is consistent with a recent meta-analysis that found a 24% increased risk of any coronary event in shift workers despite significant heterogeneity across 28 studies.
Vetter said the finding of waning CHD risk after stopping shift work needs to be replicated in other studies but that clinicians already know a lot about CVD prevention through factors such as good diet, exercise, and smoking cessation.
"I've done a lot of work in factories, and it's so hard to see that in the canteens and everywhere the diet can be so poor, and we know that the contribution of diet is so important."
The authors reported no relevant financial relationships.