Night shift work and risk of colorectal cancer: a prospective cohort study among 56 477 female nurses in the Netherlands.

Objectives: Night shift work has been classified as probably carcinogenic to humans, possibly related to suppression of melatonin secretion. Although experimental studies suggest that melatonin inhibits intestinal tumor proliferation, epidemiological evidence for a relationship between night shift work and colorectal cancer (CRC) risk is lacking.

Methods: We prospectively examined the association between night shift work and CRC in the Nightingale Study. We included 56 477 Dutch female nurses who completed a questionnaire in 2011, including occupational history with detailed (historical) information on night shift work. Until February 2025, 468 incident CRCs were recorded. Age-adjusted HRs and 95% CIs for associations between night shift work variables and CRC risk were estimated using Cox regressions.

Results: CRC risk did not differ between nurses who ever or never worked night shifts (HR=1.13; 95% CI=0.89 to 1.44). A longer duration of working night shifts (≥20 years: HR=1.19; 95%CI=0.89 to 1.60) was neither associated with CRC risk. However, a higher average number of consecutive night shifts per month (continuous per shift; HR=1.04, 95%CI=1.00 to 1.07) and a higher cumulative number of performed night shifts (continuous per 250 shifts: HR=1.02; 95%CI=1.00 to 1.04) were associated with a slightly increased risk. Chronotype did not affect the associations of CRC risk with night shift work.

Conclusions: Although a longer duration of night shift work was not associated with CRC risk, both a higher frequency of and a higher cumulative exposure to night shifts were associated with slightly higher risk, suggesting a potential association between extensive exposure to night shift work and CRC risk.