In Bangladesh, extreme heat is still considered a daytime event: heat waves, a blazing sun, a thermometer crossing 36°C, an official warning that expires with the evening.
But science says the deeper rupture begins after dark — when the city, the body and the workers that keep it running are supposed to recover, but cannot.
According to an analysis of 40-year ERA5-HEAT data from 1985 to 2024, Bangladesh's minimum nighttime heat stress, measured through the Universal Thermal Climate Index (UTCI), rose by 0.03°C per decade — three times the 0.01°C rise in daytime maximum extremes.
Published in PLOS Climate journal in August 2026, the study also found that across western and southern Bangladesh, cumulative nocturnal warming reached 1.0°C to 1.7°C.
Days and nights with no thermal stress have fallen below 20 a year in major urban centres. Dhaka and Rajshahi now experience more than 150 days above 38°C on the UTCI scale.
"About 70% of nighttime hours in Bangladesh carried no heat stress in 1985. By 2024, Dhaka's comfortable window had fallen to 36.5%," M Kamruzzaman Milon, the study's lead author and head of the Climate Smart Agriculture Lab at the Bangladesh Rice Research Institute (BRRI), told TBS.
"The fastest intensification appears between 5am and 7am — precisely when ambient conditions and core body temperature should be at their lowest," he added.
Why Dhaka becomes hottest after sunset
Dhaka does not simply remain hot at night. Its urban heat island reaches maximum intensity at 9pm, according to Abeda Tabassum and colleagues in the Asia-Pacific Journal of Atmospheric Sciences, in their paper "Simulating urban heat islands and local winds in the Dhaka metropolitan area, Bangladesh".
Concrete, asphalt and glass absorb solar radiation through the day and release it after sunset, while dense construction blocks wind. Central Dhaka remains 2.5°C to 5°C hotter than rural surroundings past midnight.
For poorer households, the building itself becomes a heating device. Audits by the International Institute for Environment and Development (IIED) and Ovibashi Karmi Unnayan Program (OKUP) in Korail and Kamrangirchar found that tin-roofed rooms were up to 9°C hotter than outdoors between 4pm and 10pm. They cooled by only 0.5°C an hour, remaining between 35°C and 38°C beyond 2am.
A ceiling fan cannot remove stored heat. It only accelerates air across the skin in a room whose walls and roof are still radiating energy, turning it into a domestic convection oven rented by people who have nowhere cooler to go.
The cellular bill arrives before dawn
Restorative slow-wave and REM sleep require core body temperature to fall by roughly 0.5°C to 1°C. Once bedroom temperatures exceed 24°C, the autonomic nervous system begins to strain. Heart-rate variability falls and the body remains in sympathetic overdrive.
"Bangladesh's Labour Act does not classify thermal stress as an occupational hazard. There are no enforceable heat-rest cycles, hydration guarantees, or wage protection when work becomes unsafe."
M Kamruzzaman Milon, head, Climate Smart Agriculture Lab, Bangladesh Rice Research Institute
Blood pressure should decline by 10% to 20% during sleep. In sustained heat, it may not — a pattern known as non-dipping hypertension. Continuous sweating without rehydration also concentrates the blood. The European Heart Journal links extreme nighttime heat to higher early-morning ischaemic stroke risk, as thicker blood makes clots more likely.
The kidneys pay another bill. Daytime dehydration causes microvascular and tubular strain; night should permit repair. In a humid 34°C room, sweating continues and repair does not. Repeated acute kidney injury can become irreversible fibrosis and chronic kidney disease of non-traditional origin, or CKDu — including among workers in their thirties.
An IIED-OKUP study titled "No escaping the heat: health and economic impacts of heat stress on informal workers in urban Bangladesh" found that 23.1% of workers show symptoms consistent with acute kidney injury (AKI), and around 6%, or 1 in 17, may eventually develop chronic kidney disease (CKD) from repeated heat exposure. The finding turns "poor sleep" into something far more serious: a pathway from a hot room to permanent organ damage.
The Tk50,000 crore wage drain
The biological damage reappears as an economic loss. IIED and OKUP estimate that extreme heat removes Tk50,630 crore, or $4.1 billion, from informal workers' wages each year — between 0.91% and 1.4% of GDP. Outdoor workers lose about 24 workdays annually; indoor workers lose around 15.
The sequence is brutally ordinary. Broken sleep produces morning fatigue. Fatigue slows a construction shift, cuts a rickshaw puller's trips or increases the risk of a factory accident. Fewer trips and missed shifts mean less cash that evening. With no paid sick leave, the worker absorbs the loss through smaller meals, depleted savings or new debt, including costly microcredit taken simply to cover groceries.
"When workers slow down in the heat, they silently bear the losses themselves," Ritu Bharadwaj, director of climate resilience, finance and loss and damage, IIED, said in her research.
Heat is therefore not only reducing productivity, it is transferring the cost of a warming city from employers and public institutions to the bodies and household budgets of the people least able to finance it.
The warning system stops at sunset
The Bangladesh Meteorological Department (BMD)'s heatwave warning is triggered by daytime maximum temperature of at least 36°C. It does not recognise a tropical night, when the minimum remains above 25°C, and does not fully account for humidity, wind or radiation.
A windless 35°C night at extreme humidity may impose greater physiological stress than a hotter but drier, breezier afternoon. The warning system cannot see that difference.
"Bangladesh's Labour Act does not classify thermal stress as an occupational hazard. There are no enforceable heat-rest cycles, hydration guarantees, or wage protection when work becomes unsafe," Kamruzzaman noted.
"BMD must move to ISO-standardised measures such as UTCI and Wet Bulb Globe Temperature, then connect those readings to decisions about work, school and public health," he argued.
The built environment is another policy choice. Bangladesh Poribesh Andolon (BAPA) and architect Iqbal Habib have criticised the destruction of water bodies and ventilation corridors under RAJUK and the Detailed Area Plan. The Bangladesh National Building Code offers little protection to informal homes. Cool roofs, protected wind corridors, hydration points and insulation would not eliminate heat, but would change who must absorb it.
M Zakir Hossain Khan, co-founder and managing director of Change Initiative, proposes linking WBGT or UTCI triggers to automatic payments through NID-linked services such as bKash and Nagad. A $1.62 daily wage floor during alerts would cost an estimated $31.3 million to $93.8 million a year — a fraction of the $4.1 billion already lost.
That proposal exposes the real governance failure. Bangladesh has digital payment rails, weather data, labour regulation and urban planning agencies. What it lacks is an institution required to treat an uncooling night as a health emergency, an occupational danger and an economic shock at the same time.
Because, until the night time heat stress is acknowledged, the toll would keep piling up.