Background: Animal and experimental studies suggest circadian disruption increases colorectal cancer risk, but evidence in humans is limited. We examined night shift work, chronotype, and residential position within a time zone, proxies for circadian disruption, in relation to colorectal cancer risk. Methods: Participants in the Black Women’s Health Study, a prospective cohort of 59,000 Black American women established in 1995, reported history of night shift work and chronotype on follow-up questionnaires. Residential position within a time zone was estimated using participant addresses at each questionnaire cycle. Number of colorectal cancer cases and follow-up duration varied by analysis depending on timing of exposure assessment, ranging from 204 over the 2005-2018 night shift work study period to 452 over the 1995-2018 residential position study period. Cox proportional hazards regression was used to estimate multivariable-adjusted hazard ratios (HR) and 95% confidence intervals (CI). Results: Compared to never having worked a night shift, working a night shift for ≥10 years was associated with increased colorectal cancer risk (HR=1.64, 95% CI 1.01-2.66). However, shorter duration was not. The HR for evening vs. morning chronotype was 0.96 (95% CI 0.73-1.27). Westward position of residence within a time zone was not associated with colorectal cancer risk (HR per 5-degree longitude increase: 0.92, 95% CI 0.82-1.03). Conclusions: Our findings suggest a possible increased risk of colorectal cancer associated with long duration night shift work; however, results require confirmation in larger studies. Impact: Circadian disruption from long-term night shift work may contribute to colorectal cancer development in Black women.

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