Mortality risk and vulnerability to hot nights in mainland China.

Although high temperatures pose major public health risks, limited research has focused on nighttime heat, especially in developing countries. Evidence on the health impacts of hot nights for vulnerable groups remains scarce. We conducted a nationwide time-series study using data from China's Disease Surveillance Point system between 2015 and 2019 (May-September). Hot nights, defined as daily minimum temperatures above the 95th percentile, were assessed using hot night excess (HNE) and hot night duration (HND). We evaluated all-cause and cardiovascular mortality risks associated with hot night exposure and identified vulnerable populations through stratified analyses. Hot nights significantly increased both all-cause and cardiovascular mortality across mainland China, with peaking immediately and lasting up to five days. Older adults and females were particularly susceptible. These findings highlight the urgent need to address the health impacts of nighttime heat, especially among vulnerable populations, and support the development of targeted strategies to reduce heat-related mortality.