Nigeria is experiencing rising temperatures, frequent heat extremes, and increasing exposure to thermal stress. This study is the first to offer a comprehensive, long-term (1950-2023), and spatially comparative analysis of heat stress in Nigeria using four globally recognized thermal indices, Humidex (HD), Normal Effective Temperature (NET), Wet Bulb Globe Temperature (WBGTs), and Universal Thermal Climate Index (UTCI), enabling a more nuanced understanding of regional and index-specific sensitivities to heat extremes across diverse climatic zones. Heatwave events were analyzed based on durations (1-day, 3-day, and 5-day) and periods (daytime, nighttime, and 24-hour mean) for four stress categories: moderate, strong, severe, and extreme. Findings reveal significant variations in heat stress frequency across indices, regions, and event durations. NET consistently recorded the highest number of heat stress events, particularly in northern and highland areas, highlighting its sensitivity to topographical influences. For 1-day nighttime events, NET identified up to 40 strong occurrences in southern Nigeria, while WBGTs recorded over 25 moderate cases in both regions. In consecutive 3-day events, NET reported up to 20 severe cases in northern and southern Nigeria, while WBGTs captured up to 16 extreme daytime events. For prolonged 5-day heatwaves, NET and WBGTs detected up to 12 extreme events, underscoring their responsiveness to extended heat exposure. Conversely, UTCI remained the most conservative for all durations, reflecting its strict physiological threshold. Regional analysis confirms that northern Nigeria experiences a higher frequency of severe and extreme heat stress due to its arid climate and prolonged exposure to solar radiation. Additionally, highland regions, such as Mambila and Plateau, consistently reported elevated event counts by NET and UTCI, demonstrating their effectiveness in capturing microclimatic variations. These results highlight the growing prevalence of heat stress events across Nigeria and the need for tailored heat risk assessments.