1. Night shift work was associated with a modestly increased risk of coronary heart disease in the Nurses’ Health Study and Nurses’ Health Study II.
2. Duration of night shift work was positively correlated with risk of incident coronary heart disease.
Evidence Rating Level: 1 (Excellent)
Study Rundown: Night shift work has been associated with negative health outcomes, including coronary heart disease (CHD). The Nurses’ Health Study (NHS) and Nurses’ Health Study II (NHS II) are large prospective cohort studies comprised of female nurses in the US who complete questionnaires every two years. This study used data from NHS and NHS II to explore the relationship between night shift work and incident CHD over 24 years. After adjusting for known CHD risk factors, night shift work was associated with an increased risk of CHD. Women who worked fewer than five years of night shift work had no increased risk of CHD, but women working 5 or more years in the NHS and 10 or more years in the NHS II had a significantly increased risk of CHD. Strengths of this study include the large number of participants and use of 24 years of prospective data. All of the study participants are well-educated females from the United States, and most women were Caucasian, limiting generalizability of the results. Ultimately, this study demonstrated a modest increased risk of CHD among shift workers, suggesting that more research should be done to explore the health impacts of shift work.
In-Depth [prospective cohort]: This study utilized 24 years of prospective data from NHS and NHS II to examine the risk of CHD among night shift workers. In the NHS, night shift work for fewer than 5 years was associated with an HR of 1.02 (95% CI 0.97-1.08), night shift work for 5-9 years was associated with an HR of 1.12 (95% CI 1.02-1.22), and night shift work for 10 or more years was associated with an HR of 1.18 (95% CI 1.10-1.26) of CHD (p for trend <0.001). In the NHS II, night shift work for fewer than 5 years was associated with an HR of 1.05 (95% CI 0.97-1.13), night shift work for 5-9 years was associated with an HR of 1.12 (95% CI 0.99-1.26), and night shift work for 10 or more years was associated with an HR of 1.15 (95% CI 1.01-1.32) of CHD (p for trend=0.01).
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