Association between outdoor light-at-night exposure and colorectal cancer in Spain (MCC-Spain study).

BACKGROUND Night shift work, exposure to artificial light-at-night and particularly blue light spectrum, and the consequent circadian disruption may increase the risk of breast and prostate cancer. Colorectal cancer risk may also be increased among night-shift workers. We investigated the association between exposure to artificial light at night according to light spectrum and colorectal cancer among subjects who had never worked at night in a general population case-control study in Spain. METHODS We examined information on 661 incident histologically verified colorectal cancer cases and 1322 controls from Barcelona and Madrid, 2007-2013. Outdoor artificial light at night exposure was based on images from the International Space Station (ISS) including data on remotely sensed upward light intensity. We derived adjusted odds ratio (OR) estimates and confidence intervals (CI) for visual light, blue light, and spectral sensitivities of the five human photopigments assigned to participant's geocoded longest residence. RESULTS : Exposure to blue light spectrum was positively associated with colorectal cancer (OR=1.6; 95%CI: 1.2-2.2; highest vs. lowest tertile). ORs were similar (OR=1.7; 95%CI: 1.3-2.3) when further adjusting for area socioeconomic status, diet patterns, smoking, sleep and family history. We observed no association for outdoor visual light (full spectrum) (OR = 1.0, 95%CI 0.7-1.2; highest vs. lowest tertile). Analysis of the five photopigments gave similar results with increased risks for shorter wavelengths overlapping with the blue spectrum and no association for longer wavelengths. CONCLUSIONS Outdoor blue light spectrum exposure that is increasingly prevalent in recent years may be associated with colorectal cancer risk.