Effects of hot nights on mortality in South Europe

Background: There is strong evidence concerning the impact of heat stress on mortality, particularly from high temperatures. However, few studies emphasise the importance of hot nights. In cases of high night temperatures, thermal stress persists and is aggravated by the fact that the human body is prevented from nocturnal rest. The effect of nighttime thermal environment on mortality has been recently explored using different approaches. Objectives: In this study, we assess the efficacy of using hot night duration and hot night excess to predict daily cause-specific mortality in summer, using multiple cities in several countries in Southern Europe. Methods: We fitted time series regression models to summer cause-specific mortality, including natural, respiratory and cardiovascular causes, in 11 cities across four countries. We included a distributed lag non-linear model with lags up to 7 days for Hot Night duration [HNd] and Hot Night excess [HNe] adjusted by daily mean temperature. City-specific associations were summarised as overall-cumulative exposure-response curves at the country level using meta-analytic techniques. Results: We found a positive association between the relative risk of cause-specific mortality and HNd and HNe with a non-linear convex relationship in most cities; the higher the duration or excess, the higher the risk of mortality. The effects of HNd were only significantly associated in Portugal with risks of 1.29 (95% confidence interval [CI], 1.07 to 1.54) for nonaccidental