Assessing the relationship between outdoor light at night (using International Space Station photographs) and risk of breast cancer in Vancouver, British Columbia

Abstract

Background/Aim: Chronobiological and epidemiologic studies suggest that exposure to light at night (LAN) disrupts circadian rhythms, which in turn may increase breast cancer risk. The aim of this study is to investigate how different spectral parameters of outdoor LAN, as measured by the International Space Station (ISS), are associated with breast cancer risk, particularly in terms of the blue part of the spectrum. Methods: A population-based case-control study was conducted in Vancouver, BC, Canada with incident breast cancer cases and controls frequency matched by age. This analysis was restricted to 687 cases and 719 controls who provided lifetime residential histories. Using time-weighted average duration at each home 5-20 years prior to study entry, measures of cumulative average outdoor LAN exposure were estimated using ISS photographs, with a total of six LAN measures of intensity and varying spectra. Outdoor LAN measures were categorized into tertiles, and logistic regression was used to estimate the associations between outdoor LAN and breast cancer risk and potential interactions by menopausal status and night shift work. Results: Most participants lived at one or two residences during the 5-20 year time period of interest prior to study enrolment (median of two residences, maximum of eight residences). Preliminary results do not suggest an association between the various spectral LAN measures and breast cancer risk, and there is no clear interaction by menopausal status or past/current night work status. Conclusions: Our preliminary findings are consistent with no relationship between spectra and intensity of outdoor LAN measured using ISS photographs of Vancouver and risk of breast cancer. Strengths include the use of images with higher resolution around residences, and analysis of outdoor LAN using various biologically relevant parameters. More work is needed to understand the specific mechanisms linking LAN to breast cancer risk, particularly in terms of blue light exposure.