The heat of the night: the impact of nocturnal heat excess on mortality in Brazilian state capitals (2000–2019)

Abstract Background:Nighttime heat events are increasing in frequency and intensity, but studies on their mortality impacts focus mainly on the Northern Hemisphere, overlooking the Global South. Brazil's climatic and socioeconomic diversity offers a unique context to explore this association.Objectives:This study aimed to analyze the influence of nighttime heat on all-cause, cardiovascular and respiratory disease mortality across Brazil and determine whether demographic, geographic, and socioeconomic variables affect the exposure-response curve and heterogeneity of results.Methods:This study analyzed data from all 26 state capitals and the federal district of Brazil, spanning 2000 to 2019. Nighttime heat was measured using the Hot Night Excess (HNE) metric: the excess of hourly nighttime temperatures above the 95th percentile of daily minimum temperatures. A three-step analysis was performed: a city-specific time-series models using Generalized Linear Models with Distributed Lag Non-Linear Models; a random-effects meta-analysis of pooled city estimates; and a meta-regression with socio-demographic and geographic factors.Results:A total of 5,537,177 all-cause deaths, 1,559,878 CVD deaths, and 619,558 RD deaths were analyzed. HNE was associated with significant increases in all-cause (relative risk (RR) = 1.19,95%CI: 1.09–1.30) and CVD mortality (RR = 1.20, 95%CI: 1.07–1.34), while RD mortality showed anonsignificant increase (RR=1.27, 95%CI: 0.94–1.71). Regional analyses revealed stronger associationsin the South, Southeast, and Midwest, with weaker or nonsignificant associations in the North andNortheast. Meta-regressions demonstrated that closer to the equator latitudes were associated withhigher RRs for all-cause mortality.Discussion:The study highlights regional differences in the impact of nocturnal heat on mortality, probably driven by acclimatization, infrastructure, and socioeconomic factors. These findings underscore the need for tailored public health strategies and climate adaptation to address regional vulnerabilities.