Global warming increases understanding of human stress from heat, notably in tropical countries like Nigeria. Research examines mean physiological thermal stress in Nigeria with UTCI. ERA5 data from ECMWF for 84-year period (1940-2024) analyzed alomg with1940, 1970, 2000 and 2024 .The study shows a notable and clear spatial variation in thermal stress throughout Nigeria, with the most severe stress observed in Anambra state reaching 32°C in 2024, and the mildest in Katsina state at 25°C during the same year. The results suggest a range of Universal Thermal Climate Index (UTCI) from 26 to 32°C in Nigeria, indicating moderate heat stress. These findings have implications for public health, agricultural output, and occupational safety. This paper underscores the necessity of incorporating advanced bioclimatic indices like UTCI into national climate adaptation and public health policies to develop targeted interventions for the most at-risk populations.