Daytime and nighttime heatwave intensity and acute care utilization for mental and neurological disorders in California

Heatwave exposures have been linked to a variety of mental and neurological disorders. Little is known, however, about the potentially differential associations of daytime versus nighttime heatwave intensity with subtypes of mental and neurological disorders. In this time-stratified case-crossover study, we estimated and compared the associations of typically dry daytime and typically humid nighttime heatwave intensities, characterized by heatwave indices (HWIs), with acute care utilizations for various subtypes of mental and neurological disorders in 1412 ZIP Code Tabulation Areas in California from 2006 to 2019. A total of 4309 294 acute care utilizations for mental disorders and 2097 563 for neurological disorders were included in this study. Higher associations with nighttime HWI were found for most disease subtypes, including anxiety disorder, depressive disorder, schizophrenia, bipolar disorder, post-traumatic stress disorder, Alzheimer's disease and related dementias, and Parkinson's disease; while daytime HWI showed a higher impact on conduct disorders (<i>P</i> < .001). On average, during the warm season in California, nighttime heatwaves accounted for about 70.6% and 34.0% of acute care utilizations for mental and neurological disorders that were attributable to heatwaves, respectively. Our findings highlight the detrimental impacts of humid nighttime heatwaves on mental and neurological health and call for innovative heat preparedness actions and increased awareness among public health practitioners as more nighttime heatwaves are anticipated under climate change.