Daytime, nighttime, and day-night compound heatwaves and the risk of depression: A Chinese nationwide cohort.

Despite growing population and animal evidence linking mental health to environmental stressors, the potentially differential relationships of daytime and nighttime heat exposure with depression have been underexplored in both developed and undeveloped settings. This study aimed to investigate the long-term associations of daytime, nighttime, and day-night compound heatwaves with depressive symptoms in a Chinese nationwide cohort of middle-aged and older population. We analyzed 35211 repeated-measurement observations from 12403 participants (≥45 years) across 125 cities using four waves (2011-2018) of China Health and Retirement Longitudinal Study data. Daytime, nighttime, and compound heatwaves were defined based on multiple temperature thresholds (P90, P92.5, P95) and durations (2-4 consecutive days) for each city. Depressive symptoms were assessed using the 10-item Center for Epidemiologic Studies Depression Scale. Logistic mixed-effects models were employed to quantify heatwave-depression associations and to curve exposure-response functions using restricted cubic spline smoothing. We did several subgroup analyses to testify potential effect modifications by demographic and environmental factors. In both unadjusted and adjusted models, we observed parallel evidence for stronger associations of depressive symptoms with nighttime and compound heatwaves than daytime heatwaves under multiple definitions. Exposure-response analyses demonstrated approximately linear threshold-free associations for daytime heatwaves but J-shaped patterns for nighttime and compound heatwaves. In subgroup analyses, we observed significantly greater vulnerability to heatwave-associated depression among younger (under 65), high-educated, and urban populations, as well as those residing in high air pollution and low-temperature communities, suggesting potential air pollution-climate interactions in elevating heat-related depression risks. Our study provided novel evidence for greater associations of depressive symptoms with nighttime and compound heatwaves than daytime heatwaves. These findings highlighted the public health urgency to mitigate mental health risks posed by nocturnal and diurnal heat events in the context of global climate warming.