Breast cancer is the second leading cause of cancer death among US women.¹ Black US women, who are disproportionately burdened by breast cancer mortality, are 40% more likely than White women to die from the disease.¹ The Black-White disparity is even higher in some states, including Georgia, where breast cancer is the leading cause of cancer death among Black but not White women.² Many factors contribute to this disparity, including differential access to quality treatment, high prevalence of comorbidities, and unfavorable tumor characteristics.³ Despite this knowledge and recent declines in mortality rates, the breast cancer mortality racial disparity persists.¹

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 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). Results: 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). 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.

U.S. racial and ethnic minoritized groups face disproportionate cancer burdens compared to White Americans. Investigating modifiable factors, such as sleep, that are socially patterned and inequitably distributed by race and ethnicity may advance understanding of cancer disparities and provide intervention opportunities. Emerging data suggest poor sleep health is associated with cancer. Yet, its contribution to racial and ethnic cancer disparities is understudied. In this narrative review, we explored the sleep–cancer relation through a disparities lens. We (i) summarized literature reporting on associations between sleep and cancer among racial and ethnic minority populations; (ii) examined potential sleep–cancer mechanisms; and (iii) discussed future directions. We identified five studies reporting on sleep–cancer associations among minoritized groups. Poor sleep health was associated with aggressive breast cancer among Black women, increased breast cancer risk among Asian women, and increased risk of breast and total cancer among Hispanic/Latinx Americans. Sleep and cancer disparities have similar socioeconomic and behavioral determinants, suggesting racial and ethnic minoritized groups may be vulnerable to poor sleep health and its adverse health impacts. Evidence indicates that the sleep–cancer disparities relation is an emerging, but important area of research that warrants further investigation, as sleep may be an avenue for reducing cancer disparities.