Bedrooms that become furnaces: Rising temperatures plague Indian cities

NEW DELHI/BENGALURU – Come May, Durga Devi and her two teenage daughters know it is time to give up the familiar comfort of sleeping on their bed. For the past five years or so, with the onset of each summer in Delhi, the 45-year-old and her children have spent their nights on the terrace atop their one-room flat.

Instead of sleeping in their windowless bedroom, which transforms into a furnace on hot nights, the family prefer sleeping on the terrace floor where they can catch some breeze, and have installed an air cooler. “But on days when it gets too hot, we can’t even sleep there,” said Devi. “The air from the cooler feels hot; it is as if the cooler is not working at all.”

With inadequate sleep at night, she struggles at times to stay alert at her sari-making workplace, where a single ceiling fan churns the hot air for around six workers. “By 1pm on some days, I feel restless, my blood pressure falls, and I feel dizzy,” she said.

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Devi’s experience highlights an overlooked heat-related challenge – nights in India are getting warmer, exposing working-class families to ongoing heat stress even after a hot day and affecting their health in the long run.

A 2025 report by the Council on Energy, Environment and Water, a Delhi-based think-tank, found that 35 out of 36 Indian states and union territories had witnessed rising night-time temperatures.

It also reported that nearly 70 percent of Indian districts experienced at least five additional very warm nights each summer from 2012 to 2022, while about 28 percent of districts saw a similar increase in very hot days.

Delhi, for instance, experienced its warmest May night in nearly 14 years on May 25, 2026, with an overnight low of 32.4 deg C. And the all-time record for Delhi’s highest minimum temperature in June is 35.2 deg C, recorded on June 19, 2024.

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Temperatures begin to fall with the onset of the monsoon, which covers most of the country by July, but hot nights can still feel oppressive due to increased humidity.

“The rise in night-time temperatures is a grave issue and should be treated on a par with daytime temperatures,” said Kartiki Negi, the lead for climate impacts at research-based consultancy Climate Trends.

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“The effects from high night temperatures are not as visible to the eye, unlike during the day when one goes out and sweats in the sun,” she told The Straits Times.

It is Devi’s 18-year-old son Abhishek who bears the brunt of the night-time heat. He sleeps in their room instead of the terrace, to keep watch on their meagre belongings, and gets barely any sleep.

“The bed heats up so much that I cannot sleep,” said Abhishek, who is part of a Greenpeace India “heat registry” effort to document the impact of heat-related stress on families in certain parts of Delhi.

He takes temperature readings inside using a handheld thermal camera, which has recorded highs in June in the mid-30s or even higher around midnight. To find some relief, he gets up every now and then to splash his face and arms with water.

Warmer nights are a concern as they prevent the human body from cooling down and recovering from the stress it experiences during daytime heat. This is especially challenging for the urban poor and middle-class residents like Devi who live in poorly ventilated homes made of heat-trapping materials such as tin roofs or concrete.

More often than not, such families also have poor access to cooling measures or a reliable electricity supply, making them disproportionately affected by hotter nights.

Over time, this cyclical day-night heat stress significantly increases risks such as heat stroke and worsens non-communicable diseases such as diabetes and hypertension because of the strain on the body’s systems and dehydration, among other reasons.

READ: Heatstroke kills 16 in India as temperatures climb

“When night-time temperatures do not fall and instead keep increasing, it also creates a vicious circle because then the morning after starts with further elevated temperatures, enabling daytime temperatures to rise even further,” said Negi.

A key reason for warmer nights in Indian cities is the rapid and dense growth in construction using concrete, a material that absorbs heat during the day and releases it at night.

This phenomenon, known as the urban heat island effect, is aggravated by reduced green cover in urban areas, with fewer trees and open spaces leading to less natural cooling. Bengaluru, for instance, has lost over 90 percent of its lake and forest cover since the 1970s.

A greater presence of greenhouse gases and aerosols such as dust or soot in the atmosphere also prevents the heat that the earth releases at night from escaping.

Velu Vinoj, an associate professor at the School of Earth, Ocean and Climate Sciences at the Indian Institute of Technology in Bhubaneswar, said there is another factor – water vapor – that aggravates rising night-time temperatures, especially in cities closer to the coast, where there is more moisture.

“Water vapor is a greenhouse gas, which traps heat,” Vinoj told ST. “Not only that, it can actually wet air pollution particulates, allowing them to trap even more heat.”

In Mumbai, a city on the coast of the Arabian Sea, it got so hot in June that people from some crowded neighbourhoods with cramped and poorly ventilated homes were forced to sleep in the open at Versova Beach.

In Chennai, a tropical coastal city, homemaker A. Induja, who lives in a two-room house with a tin-sheet roof, pours buckets of water on the floors of her first-floor rented home in the evening to try to cool the space.

“I also have a bath in the evening to cool off. I wear a light cotton nightie and my husband wears a lungi and vest – anything to let the skin breathe,” the 33-year-old told ST.

The family has taken to sleeping on the bedroom floor, which is a little cooler, on cane mats. They use a standing fan, but it “only circulates hot air”, Induja said. She sleeps fitfully at night, uncomfortably warm, until she wakes up, still tired, at about 6am.

“In the day, I go sit under the single neem tree outside where it is cooler even if not a leaf moves. But at night at home, there is no breeze. We feel trapped in a hot box, but it is the only house (whose 5,000 rupees or S$70 monthly rent) we can afford,” she said.

After sweltering months starting from February, only the rains in July brought some relief, but even then, Induja said it remains hot indoors most of the year.

READ: India’s heatwave longest ever, worse to come

A May 2026 study that tracked indoor heat in low- and middle-income houses in Chennai, including Induja’s, found that temperatures remained above 34 deg C for hours after sunset, even in the cooler months of December and January – due to the urban heat island effect – when temperatures should be below 30 deg C. Worse, relative humidity remained above 75 percent throughout, amplifying the effects of heat on the body by reducing sweating, its primary mechanism for cooling.

Prolonged heat exposure causes thermal discomfort, but if it continues and a person cannot get relief from a cool breeze, sweating, air-conditioning and a good sleep to recover from the day’s heat, it develops into heat stress, the signs of which are nausea, fatigue and dizziness, said Naveen Puttaswamy, an associate professor at the Faculty of Public Health at Sri Ramachandra Institute Of Higher Education and Research, and co-author of the Chennai study.

“Women who experience long heat exposure along with poor air quality could have different birth outcomes too, a factor we are still studying,” he added.

Induja and her one-year-old in Chennai, who spend more time indoors than her mechanic husband and school-going toddler, have heat rash all over their faces and necks. She uses a prickly heat powder to treat it, but the constant sweating leaves a burning sensation.

The study conducted by Climate Trends using seven months of high-resolution sensor data found that chronic indoor heat exposure is driven by climate change, urbanization and the widespread use of heat-retaining construction materials.

To tackle this problem, Negi suggested measures like the wider use of reflective paint on roofs, cool shelters in dense working-class areas that are open at night, and neighborhood healthcare centers to respond to heat-related emergencies after sunset.

A long-ignored concern, the rise in night-time temperatures has only recently begun to get some attention in policymaking, with heat action plans of a few cities listing dedicated approaches to deal with the problem. Implementation, however, has yet to take off widely and make any meaningful difference.

Creating more urban greenery and reducing pollution are other important interventions to tackle rising night-time temperatures in cities. But with less than 40 percent of India’s population living in urban areas, the country also has an opportunity to build cities efficiently as it rapidly urbanises so that they are better equipped to deal with rising heat.

One way to do this, noted Vinoj, is to improve city-level ventilation by creating wind corridors that allow air to flow through cities along their natural pathways, especially those in coastal areas, instead of obstructing the airflow with buildings or other construction.

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“It’s very difficult to change a city that is already built, but if you plan and build new ones in such a way that you create cooling wind corridors, bringing the cool sea breeze deep into the city, it will actually take away the heat,” he said. “We have not yet built all our cities, so why don’t we build them with more efficient cooling?” /dl