On average, nights are warming faster than days across most of the United States, according to the 2018 National Climate Assessment Report. It’s part of a global trend that’s being fueled by climate change. Unusually hot summer nights can lead to a significant number of deaths, according to climate scientists and environmental epidemiologists, because they take away people’s ability to cool down from the day’s heat. “What’s making the news is the highs, but nighttime minimums have an impact on mortality,” said Lara Cushing, an environmental health scientist at the U.C.L.A. Fielding School of Public Health.