Projections of day and night heatwave impacts on mental disorder mortality: a population-based case-crossover study in Shanghai, China

Heatwaves pose a growing global health threat, yet the mental health impacts of different heatwave types under climate change remain unclear. We quantified current and future mental disorder mortality attributable to daytime, nighttime, and compound heatwaves in Shanghai, China. Baseline associations between heatwaves and daily mental disorder deaths (2016-2019) were estimated using a time-stratified case-crossover design. Heatwaves were defined using a health-based excess heat factor, and attributable fractions in the 2010s were projected to the 2030s, 2060s, and 2090s under three Shared Socioeconomic Pathways and three population adaptation scenarios. All heatwave types were associated with increased mental disorder mortality, with the strongest effects observed for daytime heatwaves, followed by compound and nighttime heatwaves. Without adaptation, excess deaths attributable to daytime heatwaves were projected to increase by 8.3%-10.7% in the 2060s and 13.6%-15.3% in the 2090s, while the full exposure-response function projected smaller increases in attributable deaths for compound and nighttime heatwaves. Schizophrenia exhibited the greatest vulnerability, particularly to daytime heatwaves, with attributable deaths rising by 27.9%-34.9% by the 2090s. Suicide and dementia were most affected by compound and nighttime heatwaves, respectively. Partial and full adaptation substantially reduced future burdens, especially for nighttime heatwaves. Enhanced population adaptation could mitigate the death burden of mental disorders from heatwaves.