Short-term association between hot nights and mortality: a multicountry analysis in 178 locations.

The increase in hot nights in the last decades and the projected increase due to climate change makes increasing knowledge about their impact important and critical for measures of public health actions and adaptation planning. However, few studies have addressed the importance of hot nights, which may prevent necessary nocturnal rest. This study estimated the association between hot-night excess (the sum of excess heat during the nighttime above a threshold) and duration (the percent of nighttime with a positive excess) and daily mortality in the warm season over multiple locations worldwide. We fitted time series regression models to mortality in 178 locations across 44 countries using a distributed lag non-linear model over lags of 0-3 days, controlling for daily maximum temperature. Next, we used a multivariate meta-regression model to pool results. We found a positive, increasing mortality risk with hot-night excess and duration. The pooled relative risks of death associated with extreme excess and duration, defined as the 90th percentile with respect to 0 in each index, were both 1.03 (95% CI, 1.02; 1.04). The overall estimated attributable fractions were also observed to be almost similar at 0.59% (95% CI, 0.00; 1.13%) and 0.58% (95% CI, 0.00; 1.10%), respectively. The magnitude of effects ranged from an RR of 1.02 to 1.04, slightly higher in tropical and arid climates.  The highest overall estimated attributable fractions with hot-night excess were found for the arid climate, at 1.11% (95% CI, 0.00; 2.35%), while the largest burden has been estimated for the tropical climate zone, at 1.19% (95% CI, 0.00; 2.38%). This study provides new evidence that hot night indices considering excess and duration were strongly associated with an increase in the risk of mortality. A clear advantage of these exposure metrics is that they more realistically reflect thermal exposure over the entire night period rather than a single-moment temperature, such as minimum temperature.  The use of hourly data allows for a more detailed assessment of the thermal characteristics of warm season nights, making it possible to accurately assess the risk of hot nights for population health and wellbeing. Modeling thermal characteristics' sub-hourly impact on mortality during the night could improve decision-making for long-term adaptions and preventive public health strategies.