Abstract: Using daily maximum and minimum temperature data from 88 national meteorological stations in Hunan Province, atmospheric circulation data from NCEP/NCAR monthly reanalysis, and NOAA monthly sea surface temperature (SST) reconstruction data (ERSST V5) from 1961 to 2025, this study analyzed the spatiotemporal variations of the day-night heat wave in Hunan and its corresponding atmospheric circulation and SST anomalies using linear correlation, wavelet analysis, and composite analysis. The results show that the number of days with day-night heat wave from June to September in Hunan increased significantly at a rate of 0.89 days/10a from 1961 to 2025. Specifically, the overall trend was decreasing from 1961 to 1997, while a highly significant increasing trend emerged from 1998 to 2025. Spatially, the number of days with day-night heat wave is higher in plains areas than in mountainous areas. Over the past 65 years, 97.7% of stations have shown a significant increasing trend, with the largest increases mainly located in the Dongting Lake area of northern Hunan, the Changsha-Zhuzhou-Xiangtan region, the eastern part of western Hunan, and the central part of southern Hunan. The key atmospheric circulation systems leading to the increase in the number of days with day-night heat wave in Hunan are the barotropic positive anomalies in geopotential height field, the stronger low-level southwest warm advection, and the weaker East Asian subtropical westerly jet stream. The SST anomalies corresponding to the interannual and interdecadal variations of day-night heat wave are similar: warm SST anomalies in the Indian Ocean, South China Sea, Northwest Pacific, and North Atlantic from the previous winter to the current spring, the development of La Ni?a type anomalies in the Pacific Ocean and warm SST anomaly in the coastal area of China during the current summer. All these SST anomalies are correlated with the increased number of days with day-night heat wave.