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.


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 to 2018 night shift work study period to 452 over the 1995 to 2018 residential position study period. Cox proportional hazards regression was used to estimate multivariable-adjusted HRs and 95% confidence intervals (CI).


Compared with 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 versus 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).


Our findings suggest a possible increased risk of colorectal cancer associated with long duration night shift work; however, results require confirmation in larger studies.


Circadian disruption from long-term night shift work may contribute to colorectal cancer development in Black women.

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