Heat-related effects on mortality have been widely analyzed using maximum and minimum temperatures as exposure variables. Nevertheless, the main focus is usually on the former with the minimum temperature being limited in use as far as human health effects are concerned. Therefore, new thermal indices were used in this research to describe the duration of night hours with air temperatures higher than the 95% percentile of the minimum temperature (Hot Night hours) and intensity as the summation of these air temperatures in degrees (Hot Night degrees). An exposure-response relationship between mortality due to natural, respiratory and cardiovascular causes and summer night temperatures was assessed using data from the Barcelona region between 2003 and 2013. The non-linear relationship between the exposure and response variables was modeled using a distributed lag non-linear model. The estimated associations for both exposure variables and mortality shows a relationship with high and medium values that persist significantly up to a lag of 1–2 days. In mortality due to natural causes an increase of 1.1% per 10% (CI95% 0.6–1.5) for Hot Night hours and 5.8% per each 10º (CI95% 3.5–8.2%) for Hot Night degrees is observed. The effects of Hot Night hours reach their maximum with 100% and leads to an increase by 9.2% (CI95% 5.3–13.1%). The hourly description of night heat effects reduced to a single indicator in duration and intensity is a new approach and shows a different perspective and significant heat-related effects on human health.