Diurnal asymmetry in heat stress intensification across Bangladesh, 1985–2024: Accelerated nighttime warming and emerging urban risk

Bangladesh’s rapidly growing cities are becoming hotter, but how heat stress is changing over the day–night cycle has remained unclear. Using 40 years (1985–2024) of hourly Universal Thermal Climate Index (UTCI) data from ERA5-HEAT, we examined long-term changes in physiologically relevant heat stress across Bangladesh and its major cities. Results show a clear day–night imbalance in warming: nighttime heat stress (UTCIₘᵢₙ) is rising faster than daytime extremes. National trends indicate increases of +0.03 °C per decade for UTCIₘᵢₙ, +0.02 °C for daily mean UTCI, and +0.01 °C for UTCIₘax, with the strongest warming occurring in the early morning hours. This signals a steady loss of nighttime cooling that people rely on for physical recovery. The most pronounced nighttime warming occurs in western and southern Bangladesh. Major cities — including Dhaka, Rajshahi, Khulna, Chattogram, and Sylhet — show additional intensification linked to urban heat-island effects. The number of Very Strong Heat Stress days (UTCI > 38 °C) has increased by 4–15 days per decade, and cities such as Rajshahi and Dhaka now experience more than 150 such days annually. Together, these findings indicate a transition from occasional heat extremes to persistent, 24-hour heat stress, increasing risks to health, labor productivity, and urban resilience. By identifying when heat stress is rising fastest and where it is concentrated, this study provides evidence to support city-specific heat-action plans, early-warning systems, and climate-responsive urban design in rapidly warming regions.