Outdoor light at night, air pollution and depressive symptoms: A cross-sectional study in the Netherlands

Background: Artificial light at night (ALAN) may be an anthropogenic stressor for mental health disturbing humans' natural day-night cycle. However, the few existing studies used satellite-based measures of radiances for outdoor ALAN exposure assessments, which were possibly confounded by traffic-related air pollutants. Objectives: To assess 1) whether living in areas with increased exposure to outdoor ALAN is associated with depressive symptoms; and 2) to assess the potential confounding effects of air pollution. Methods: We used cross-sectional data from people (N = 10,482) aged 18-65 years in the Netherlands. Depressive symptoms were assessed with the Patient Health Questionnaire (PHQ-9). Satellite-measured annual ALAN were taken from the Visible Infrared Imaging Radiometer Suite. ALAN exposures were assessed at people's home address within 100 and 600 m buffers. We used generalized (geo)additive models to quantify associations between PHQ-9 scores and quintiles of ALAN adjusting for several potential confounders including PM2.5 and NO2. Results: Unadjusted estimates for the 100 m buffers showed that people in the 2nd to 5th ALAN quintile showed significantly higher PHQ-9 scores than those in the lowest ALAN quintile (B-Q2 = 0.503 [95% confidence intervals (CI): 0.207-0.798], B-Q3 = 0.587 [95% G: 0.291-0.884],B-Q4 = 0.921 [95% CI: 0.623-1.218], B-Q5 = 1.322 [95% Cl: 1.023-1.620]). ALAN risk estimates adjusted for individual and area-level confounders (i.e., PM2.5, urbanicity, noise, land-use diversity, greenness, deprivation, and social fragmentation) were attenuated but remained significant for the 100 m buffer (B-Q2 = 0.420 [95% CI: 0.125-0.715].B-Q3 = 0.383 [95% CI: 0.071-0.696], B-Q(4) = 0.513 [95% CI: 0.177-0.850], B-Q5 = 0.541 [95% CI: 0.141-0.941]). When adjusting for NO2 per 100 m buffers, the air pollutant was associated with PHQ-9 scores, but ALAN did not display an exposure-response relationship. ALAN associations were insignificant for 600 m buffers. Conclusion: Accounting for NO2 exposure suggested that air pollution rather than outdoor ALAN correlated with depressive symptoms. Future evaluations of health effects from ALAN should consider potential confounding by traffic-related exposures (i.e., NO2). (C) 2020 The Authors. Published by Elsevier B.V.