Light at Night Exposure and Breast Cancer Risk: A Matched Case-Control Study in England

Abstract

Background/Aim: There is evidence that light at night (LAN) is a breast cancer risk factor, with in vitro and in vivo studies showing LAN can interrupt melatonin production, a hormone with oncostatic properties. Epidemiological evidence is limited, but a case-control study in Georgia, United States, using lung cancer controls, found LAN exposure to be associated with an increased risk of breast cancer. We conducted a matched case-control study to establish the association between outdoor LAN exposure and female breast cancer risk in England. Methods: Cancer data were obtained from the Office for National Statistics for years 2001 to 2012. LAN values were obtained from satellite data and assigned to each case/control’s residential postcode centroid (the geographic centre of a collection of approximately 15 adjacent households). Breast cancer cases aged ≥ 15 years were matched to controls by age at diagnosis (in years) and quintile of neighbourhood-level deprivation. The association between LAN and breast cancer was assessed using: i) skin cancer controls, ii) lung cancer controls, and iii) skin, leukaemia, brain/CNS and urinary organs (excluding kidney) cancer controls, using logistic regression, adjusted for year of diagnosis. Results: Our study included 348,561 matched breast cancer cases/skin cancer controls, 163,896 breast cancer cases/lung cancer controls, and 377,495 breast cancer cases/skin, leukaemia, brain, CNS and urinary organ cancer controls. In preliminary analyses, the risk of breast cancer was lower among women exposed to outdoor LAN when using lung cancer controls (Odds ratio in 5th vs 1st LAN quintile 0.90 (95% CI 0.88-0.92), but higher when using skin cancer controls (OR 5th vs 1st LAN quintile 1.41 (95% CI 1.39-1.43). Conclusions: Globally, breast cancer is the most commonly diagnosed cancer and it has the highest mortality rate among women, and any modifiable risk factors (such as LAN) warrant further investigation to assess possible associations.