A long-term analysis of two prospective cohort studies of initially healthy female registered nurses found an association between the duration of rotating night shift work and a statistically significant increase in coronary heart disease risk.
The trials defined rotating night shift work as at least three night shifts per month in addition to day and evening shifts.
Lead researcher Céline Vetter, PhD, of the Channing Division of Network Medicine in Boston, and colleagues published their results online in JAMA on April 26.
The researchers examined 189,158 females who were part of two nurses’ health studies. The first study started in 1976 and included U.S. nurses from 30 to 55 years old, while the second began in 1989 and included U.S. nurses from 25 to 42 years old.
For both studies, the nurses responded to questionnaires at baseline and then received biennial follow-up questionnaires to update information on their medical history, lifestyle factors and newly diagnosed diseases. The participation rates were approximately 90 percent, according to the researchers.
This analysis included 73,623 women from the first study who filled out their baseline questionnaire in 1988 and 115,535 women from the second study who filled out their baseline questionnaire in 1989.
The researchers defined coronary heart disease as angiogram-confirmed angina pectoris, coronary artery stents, angioplasty and CABG.
After 24 years of follow-up, there were 10,822 incident coronary heart disease cases. Women in the second study were younger, more likely to be nulliparous, had slightly lower alcohol consumption, reported fewer pack-years of smoking, had fewer comorbid conditions and took fewer medications and multivitamin supplements compared with women in the first study.
After adjusting for ages, women who worked less than five years of shift work did not have a significantly increased coronary heart disease risk compared with women with no history of rotating night shift work. However, the researchers said there was a significant association between longer durations of shift work and coronary heart disease risk.
In the first study, the incidence rate of coronary heart disease per 100,000 person-years was 525.7 for women working five to nine years of shift work and 596.9 for women working 10 or more years of shift work. In the second study, the incidence rate was 151.6 and 178.0, respectively.
After adjusting for multiple coronary heart disease risk factors, the elevated risk found in women working five years or more of shift work persisted in the first study and in women working 10 years or more of shift work in the second study.
In the first study, the association between duration of shift work and coronary heart disease was stronger in the first half of follow-up compared with the second half (27 percent vs. 13 percent increased risk for at least 10 years of rotating nigh shift work). In the second study, a longer time since quitting shift work was associated with decreased coronary heart disease risk.
“The results suggest that recent shift work might be most relevant, as significantly stronger associations were observed in the first vs second part of follow-up in the [first study] in addition to an association between decreasing [coronary heart disease] risk with increasing time since quitting shift work in the [second study],” the researchers wrote.
The researchers noted that results of this study could only be generalized to women. They added that unmeasured confounders could have affected the findings because of the study’s observational design.
“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 researchers wrote. “Furthermore, studying [coronary heart disease]-related biomarkers (eg, triglycerides, cholesterol levels, carotid plaque, or hemoglobin A1c) might be useful in understanding underlying mechanisms.”