0816 Actigraphy-Measured Sleep Timing and Risk of Breast Cancer in the UK Biobank

Introduction Shift work leading to circadian disruption is associated with increased breast cancer risk, but it remains unclear whether typical sleep timing variation in the general population that may cause less extreme disruption is also associated with increased risk. Prior studies relied on sleep timing assessed using self-report, but research using actigraphy is limited. Our goal was to characterize the association between sleep timing and breast cancer risk using actigraphy-based measures. Methods Our sample included 43,109 women without a history of breast cancer in the UK Biobank cohort who wore Axivity-AX3 accelerometers over 7 days. We estimated sleep midpoints, the median of sleep onset and waking. Social jetlag was estimated as the absolute difference in average weekend and weekday sleep midpoints. We used multivariable Cox models to estimate hazards ratios (HRs) and 95% confidence intervals (CI), adjusting for age, body mass index, participant characteristics, socioeconomic status, physical activity, menopause, parity, and oral contraceptive use. We conducted a sensitivity analysis excluding shift workers to control for potential strong confounding from shift work. Results Over 6.2 years of follow-up, there were 683 incident breast cancer cases. Earlier and later sleep midpoints were not associated with breast cancer (Early, < 4:00AM HR:1.08 [95%CI: 0.89-1.32]; Later, ≥5:00AM HR:1.24 [0.86-1.81]; vs. Reference 4:00-4:59 AM). Per-hour increases in social jetlag were suggestively associated with greater breast cancer risk (HR:1.09 [0.96-1.24]), but significantly higher risk was only noted at the highest levels of social jetlag comparing ≥2 hours to < 1 hour (HR:1.58 [1.13-2.20]). Results were unchanged when excluding shift workers. Conclusion Higher levels of social jetlag among UK women appeared to be associated with elevated breast cancer risk, possibly due to circadian disruption. While our results suggest that minimizing variation between weekday and weekend sleep timing may be related to lower breast cancer risk, replication of our findings in future studies using actigraphy-based measures and larger case numbers is needed. Support (if any) Supported by the Intramural Research Program of the National Cancer Institute, National Institutes of Health.