Nighttime-Accentuated Heatwaves in California: An Application of Spatial Epidemiology to Inform Policy

Extreme heat events (EHEs) are the deadliest climate-related hazard in the United States, with California bearing the highest burden of heat-related hospitalizations. While daytime heat is dangerous, climate change is increasingly driving extreme nighttime temperatures, with minimum temperatures rising more rapidly than maximums. Yet most adaptation policies focus solely on daytime conditions, overlooking the distinct health risks of sustained overnight heat. This mixed-methods study integrates spatial epidemiological mapping with stakeholder interviews to assess the health impacts and policy challenges associated with nighttime-accentuated EHEs in California. Quantitative findings show that hospitalizations linked to minimum temperature EHEs were spatially diffuse but concentrated in marginalized communities, particularly in the Southern Deserts and coastal urban areas. Interviews with state, county, and NGO stakeholders highlighted systemic gaps in policy attention, infrastructure access, and interagency coordination. Participants also identified opportunities for targeted adaptation, including revised heat alert systems, equitable retrofit programs, and expanded emergency planning. Nighttime-accentuated heat waves are not only an intensifying climatic trend but also a distinct and underrecognized public health hazard. By combining spatial epidemiology with stakeholder insight, this study reveals structural blind spots that limit the effectiveness of current heat adaptation. It advances the science of heat-health risk and demonstrates how integrated, equity-centered approaches can inform more responsive and just climate strategies. As climate change accelerates, failure to address rising nighttime temperatures will deepen health disparities and undermine existing interventions. This research calls for redefining extreme heat adaptation to include nighttime exposure and embedding that recognition into early warning systems, infrastructure planning, and long-term public health policy.