Abstract: Objective To investigate the impact of nighttime heat on cardiovascular disease mortality during warm seasons in Zibo City, and to identify patients with cardiovascular disease who are sensitive to nighttime heat. Methods The daily cardiovascular disease death data, meteorological and air pollution data in Zibo City were collected from December 31 2015 to January 1 2019. The intensity of nighttime heat was assessed using the hot night excess(HNE)index, which was calculated by determining the excess sum of high temperatures during the night. The distributed lag nonlinear model was used to analyse the impact and lag effects of nighttime heat on cardiovascular disease mortality. Subgroup analyses were performed by age and gender. Results During the study period, there were 28,079 deaths due to cardiovascular disease in warm seasons(May to September). The exposure-response relationship curve showed that HNE was associated with cardiovascular disease mortality, with a lag effect. The relative risk of extreme HNE peaked at days 0 after exposure, with RR of 1.09(95%CI: 1.03-1.15); the cumulative effect peaked at the 7th day, with RR of 1.40(95%CI: 1.27-1.54). Mortality risk was higher for ischemic heart disease and myocardial infarction than for cardiovascular disease and cerebrovascular disease. Furthermore, females and the elderly(aged ≥65 years)had a higher attributable risk. Conclusions Nighttime heat was associated with an increased risk of mortality from cardiovascular disease. Patients with ischemic heart disease, females and people aged ≥65 years were more susceptible to extreme nighttime heat. It is necessary to consider nighttime heat in heatwave warning systems and to focus on vulnerable populations. Targeted public intervention strategies should be implemented based on the hazards of nighttime heat.

Key words: Nighttime heat, Cardiovascular disease, Mortality risk, Distributed lag nonlinear model