Reorganizing Coastal Urban Heat: Nighttime Warming, Population Co-Exposure, and Built-Environment Implications

Coastal human settlements concentrate buildings, infrastructure, and populations in climate-sensitive land–sea transition zones, yet maritime proximity does not ensure lower nighttime surface temperatures. Daytime- or hotspot-bassed assessments may therefore miss changes in day–night land surface temperature (LST) contrasts and persistent population co-exposure. This study integrates day–night thermal reorganization, population-weighted LST exceedance, and co-exposure transitions across seven coastal cities in Zhejiang, China. Annual mean daytime and nighttime LST, population, and natural and built-environment covariates were harmonized on 1 km grids for 2012, 2015, 2018, 2021, and 2024. Region-wide associations were evaluated using CatBoost models with SHapley Additive exPlanations (SHAP) and spatial-block validation.. The combined share of relatively high and high nighttime-LST grids was 2.3% in 2012 and 58.2% in 2024, whereas the corresponding daytime share varied across epochs and was 13.8% in 2024. The nighttime contribution to population-weighted LST exceedance was 3.5% in 2012 and 78.2% in 2024. Between 2012 and 2024, 6753 grids entered the high-LST–high-population state, accounting for 54.7% of initially high-population-only grids with valid endpoint data; 1872 grids remained co-exposed in all five epochs. Spatially validated models identified outcome-specific environmental associations. These findings frame coastal urban heat as a reorganization of day–night LST contrasts, population overlap, and state persistence, informing spatial screening for retrofit and preventive planning.