Background: The natural light/dark cycle is the most important environmental cue for entrainment of circadian rhythms. Residents in the western vs. eastern portion of a time zone receive less light exposure in the morning and greater light exposure at night. This differential exposure to light, despite similar work and social hours, may lead to chronic misalignment of the circadian system, and impact breast cancer risk through DNA damage and endocrine disruption mechanisms. Shift work and western time zone position have been associated with breast cancer. Our aim was to prospectively examine the association between residential position in a time zone and breast cancer risk. Methods: We analyzed data from 112,663 participants (1989-2013) in the Nurses' Health Study II, a nationwide prospective cohort of US female nurses. Distance from time zone meridian was estimated using geocoded residential addresses updated throughout follow-up in a geographic information system. Self-reported breast cancer was confirmed from medical records. Estrogen receptor (ER) and progesterone receptor (PR) status were determined via immunohistochemical analysis of tissue microarrays, pathology reports, or medical records. Time-varying Cox regression models were used to calculate hazard ratios (HRs) and 95% confidence intervals (CIs) adjusted for numerous breast cancer risk factors. Results: There was no association between residential distance from time zone meridian and invasive breast cancer risk overall (n=3,950 cases; adjusted HR per 5-degree increase in longitude moving east to west within a time zone 0.96, 95% CI 0.9$31.00) or with breast cancers defined by ER/PR status. This association was not modified by shift work, chronotype, outdoor light at night, time zone, residential mobility, or menopausal status. Conclusions: Residential position in a time zone does not appear to be associated with breast cancer risk.
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