Beyond Latitude: Disentangling Meteorological Light and Urban Context in Seasonal Affective Disorder Across Timescales

Background: Latitude is associated with seasonal affective disorder (SAD) prevalence but conflates astronomical photoperiod, meteorological light, and the built environment. We examined whether winter light adds information beyond latitude and whether urban context modifies the latitude-SAD association. Methods: We combined a structured evidence synthesis with ecological reanalyses of published prevalence data. Winter surface radiation was linked to study sites. Historical GHS-POP density was averaged within 10-km buffers (primary) and 25-km buffers (sensitivity). Within-source models used N-weighted and equal-site estimates, 30,000 permutations, and leave-one-source-out analyses; artificial light at night (ALAN) was secondary. Results: Latitude and winter radiation were nearly collinear (r = -0.981). Adding radiation to latitude increased R² by only 0.006 and did not improve leave-one-source-out prediction. Direct studies linked recent light more consistently to short-term symptom severity than recurrent onset. Five multisite clusters identified the primary urbanization interaction. Estimates were negative at 10 km (β = -0.410 weighted; β = -0.440 equal-site) and 25 km (β = -0.368; β = -0.384), and remained negative after omitting any source. Permutation evidence was mixed, and ALAN did not replicate coherently. Limitations: Analyses were ecological, outcome definitions were heterogeneous, only five clusters identified the primary interaction, and the evidence synthesis was PubMed-centered. Conclusions: Available evidence does not support a simple latitude-independent meteorological-light explanation for SAD. Photoperiod and short-term meteorological light may operate at different timescales, while urban context remains an exploratory modifier requiring independent replication.