The strongest warming occurred in the early morning, reducing the nighttime cooling that people rely on for recovery.
An analysis of 40 years of hourly data found that heat stress in Bangladesh is increasing more quickly at night than during the day. The strongest rise occurred in the early morning, suggesting less nighttime cooling across much of the country.
The pattern was especially pronounced in western and southern Bangladesh and was also seen in Dhaka, Rajshahi, Khulna, Chattogram and Sylhet. The researchers say the results can help guide city-specific heat plans, warning systems and urban design, while noting that the city trends were not directly attributed to urban heat islands.
Rising nighttime heat
Using hourly Universal Thermal Climate Index data—a measure of heat stress on the human body—from 1985 to 2024, the researchers found that nighttime heat stress rose by 0.03 °C per decade nationally. Daily mean heat stress rose by 0.02 °C per decade, while the daytime maximum rose by 0.01 °C per decade. The strongest warming occurred in the early morning.
Individual grid cells showed local trends of 0.1 to 0.4 °C per decade, and nighttime heat stress increased by about 1.0 to 1.7 °C over the full 40-year period across most of Bangladesh. Warming was strongest in western and southern parts of the country. Dhaka, Rajshahi, Khulna, Chattogram and Sylhet showed additional heat-stress intensification consistent with, but not directly attributed to, urban heat-island effects at the study’s spatial scale.
The number of days with very strong heat stress, defined as a heat-stress index above 38 °C, increased by 4 to 15 days per decade. Rajshahi and Dhaka now experience more than 150 such days annually, according to the analysis.
Why the timing matters
Nighttime cooling helps people recover from daytime heat. The study’s finding that nighttime heat stress is increasing faster than daytime extremes points to a shift toward more persistent heat across the full day, rather than heat confined to occasional daytime peaks. That may increase risks for health and labor productivity and make it harder for cities to maintain resilience during hot periods.
The results identify where and when heat stress is rising fastest. The researchers say this information can support city-specific heat-action plans, early-warning systems and climate-responsive urban design in Bangladesh’s rapidly warming cities.
