Night-Shift Work and Risk of Colorectal Cancer in the Nurses' Health Study

TLDR

Night‑shift work suppresses melatonin, a hormone that inhibits intestinal cancer growth, and while it has been linked to breast cancer, its impact on other cancers was previously unclear. The study prospectively examined whether rotating night‑shift work increases colorectal cancer risk among nurses. The authors followed 78,586 nurses from 1988 to 1998, recording rotating night‑shift exposure and incident colorectal cancers. Among 602 colorectal cancer cases, women with ≥15 years of rotating night shifts had a 35 % higher risk (RR = 1.35, 95 % CI 1.03–1.77) compared to those never working night shifts, whereas 1–14 years showed no increased risk.

Abstract

Exposure to light at night suppresses the physiologic production of melatonin, a hormone that has antiproliferative effects on intestinal cancers. Although observational studies have associated night-shift work with an increased risk of breast cancer, the effect of night-shift work on the risk of other cancers is not known. We prospectively examined the relationship between working rotating night shifts and the risk of colorectal cancers among female participants in the Nurses' Health Study. We documented 602 incident cases of colorectal cancer among 78 586 women who were followed up from 1988 through 1998. Compared with women who never worked rotating night shifts, women who worked 1–14 years or 15 years or more on rotating night shifts had multivariate relative risks of colorectal cancer of 1.00 (95% confidence interval [CI] = 0.84 to 1.19) and 1.35 (95% CI = 1.03 to 1.77), respectively (Ptrend = .04). These data suggest that working a rotating night shift at least three nights per month for 15 or more years may increase the risk of colorectal cancer in women.