Impacts of hot and cold nights on mental disorders: A three-stage nationwide hospital-based time-series study with 1.2 million patients in China

•We first reveal the adverse effect of hot and cold nights on mental disorders.•The effect of hot and cold nights was greater than hot and cold days.•The attributable fraction of mental disorders due to cold was higher than heat.•Males, the elderly, and city with few air conditioners were sensitive to cold nights.