New Delhi/Bengaluru: Flimsy tin roofs balance on thin brick walls in a squatter settlement in Saraswati Kunj in Gurugram’s Sector 53. Thirty-two-year-old Manoj, who lives in one of these small, barely-ventilated rooms, hasn’t been to work in two days. That’s not something he can afford: as a construction worker, he survives on daily wages. But Manoj doesn’t have a choice.The intense heat in mid-June has given him a stomach ache, he says. Though his daily work in the scorching sun is tough, the warmer nights in the city now have made it harder to cope.“Frequent electricity outages make it difficult for me to get rest. I only have a fan and even that is insufficient in the heat sometimes,” he says.For many of Delhi’s residents, warmer-than-usual nights are aggravating the impacts of high day time temperatures. Some say they are falling sick from being unable to get a good night’s sleep for days together; headaches, bodyaches and digestive problems are some of their physical ailments. Others say it makes them more irritable, and prone to depression. Warm nights are increasing in India in both frequency and intensity, studies show. However, solutions – including short term emergency responses and longer-term ones – are available, experts say.Tin roofs in an informal settlement in Gurugram’s Sector 53. Photo: Kavya DhumeWhat are warm nights?The India Meteorological Department announces a warm night to occur when maximum temperatures are more than 40 degrees Celsius, and minimum temperatures deviate from the normal by more than 4.5°C. A departure of more than 6.4°C from normal is called a “very warm night”. But why are warm nights such a big deal?Seventy-year-old Ajay Mohammed, who runs a car repair shop in Saket, Delhi, says that he usually sleeps at around 9 or 9:30 PM, but can now only sleep by 11 PM or midnight, when it gets a bit cooler. Arif Ghalib, who repairs tyres nearby, says that the heat gives him sleepless nights.Warm nights mean less sleep. And loss of sleep is linked to a series of health concerns: negative impacts on the immune system, increased risks of cardiovascular diseases such as strokes and heart ailments, inflammation, and cognitive and psychological damage, per this study.Moreover, a study in The Lancet Planetary Health in 2022 analysed 91.8 lakh deaths in 28 cities across China, Japan and Korea and found that the relative mortality risk on days with hot nights could be 50% higher than on days with non-hot nights. Using simulations, the study predicts that by the 2100s, the frequency of hot nights would increase by more than 30% (when compared to the <20% increase in daily mean temperature) and the intensity of hot nights would increase by 50% – correspondingly increasing mortality risks.‘Crisis in the dark’When we look at nighttime temperatures, we also need to consider humidity because it is a major factor in how heat is experienced, says Amita Baviskar, Professor, Environmental Studies and Sociology & Anthropology at Ashoka University. Put simply, humidity is the amount of water vapour in the air. High humidity can prevent sweat from evaporating efficiently, robbing the human body of its cooling mechanism. The body therefore retains heat, increasing core temperatures and triggering heat stress. According to Baviskar, however, humidity has another impact too. Delhi is traditionally characterised by dry heat but with increasing humidity levels across seasons, the city now has almost a year-round population of mosquitoes. That’s several things all at once – heat, humidity, mosquitoes and often, power outages – affecting sleep on a warm night.Also read: Extreme Heat Is Costing Delhi’s Women Workers Their Health And Income“So what we are really talking about is a crisis in the dark which is taking an immense toll on people’s health…there is a physical toll,” says Baviskar. “There is a toll in terms of the ability of your mind to be alert, fresh and deal with things and think…that kind of constant stress also takes an emotional toll. People are more irritable, they are not thinking straight, quarrels are more likely to happen, relationships deteriorate. Quality of life in some ways which are quite unacknowledged, depends on the quality of sleep.”Unlike the rich, poor people don’t have facilities to decrease the heat at night and this affects sleep, says Ramankumar Jha, a watchman in Saket. “I feel depressed. My mind is filled with irritation, restlessness, and I cannot dedicate my mind to my work. What do I speak with the next person, what not to, I am not able to think; I only say odd things,” he says.Warm nights are increasing in IndiaThese impacts may become more commonplace soon, with India recording an increase in warm nights. When scientists at the Indian Institute of Technology Roorkee in 2025 analysed land surface temperature data from satellite images to look at heat extremes across the country, they found “a substantial intensification” in both the frequency and intensity of hot days and warm nights across India in recent years (2019 to 2023), when compared from 2003 to 2012. The results showed that not only daytime heat extremes but also nighttime heat extremes are big concerns. While extreme nighttime temperatures were limited to specific hotspots from 2003 to 2012, in recent years, it increased across diverse climatic regions including parts of northern and northeastern India that previously had lower nighttime temperatures, the study noted. In fact, some districts in the country experienced more than 186 warm nights annually “indicating prolonged exposure to elevated nighttime heat”, it said.“The persistent rise in nighttime temperatures aligns with the urban heat island effect, where built-up areas retain heat, preventing cooling during the night. This phenomenon disproportionately affects densely populated and urbanised areas, increasing thermal discomfort and raising the risk of heat-related illnesses,” it read.South India showed a clear increase in both daytime and nighttime temperatures in recent years. “The newly emerging hotspots in districts there indicate a change in heat stress patterns, likely driven by rapid urbanisation, deforestation, and alterations in land use. The combination of high population density, increased built-up areas, and reduced green cover has intensified the UHI effect in these regions, making them more vulnerable to extreme heat events,” the study noted.Urban areas and hotter nightsThe link between warmer nights and urban areas is hard to ignore. A report by the Council on Energy, Environment and Water (CEEW) in 2025 found that when compared to the years from 1982 to 2011, the 10 years from 2012 to 2022 witnessed nearly 70% of India’s districts experiencing an additional five very warm nights per summer (March to June). This was “most noticeable” in districts with populations of more than 10 lakh – districts that often house tier I and II cities, per the report. Between 2012 and 2022, Mumbai witnessed 15 additional very warm nights per summer, Bengaluru 11, Bhopal and Jaipur seven each, Delhi six, and Chennai, four. “This increase can be attributed to the urban heat island effect, where cities trap heat during the day and release it at night, thus increasing nighttime temperatures,” the report noted.Other aspects that play a role in how heat is experienced are housing, building interiors and construction design, says Baviskar. Not just one room tenements in the poorer parts of the city, but new styles of construction too often lack ventilation. “Buildings which have been constructed to be centrally air conditioned, their windows don’t open,” she says. “If they have a lot of glass surfaces then they are effectively creating a green house effect and the only way you have designed those buildings to be cooled is through air conditioning.” Warm nights mean sleepless nights, with headaches, body aches, digestive problems and other physical ailments for those living in informal settlements. Photo: Kavya DhumeAir conditioning is extremely energy-intensive, and you are using a lot of energy to cool the building – something that could have been done through passive cooling, she says. “Air conditioning also makes the world hotter for everybody else because it works by removing heat from inside space and throwing it out. Therefore, in modern buildings that are airconditioned, there is a much higher heat burden for people outside.”For people like Ajay Mohammed in Delhi, this is a prime concern.“Now people have been building really big buildings and fitting ACs into that which are releasing gases. And these ACs face galis and the smaller houses here are too congested to have windows. The ACs continually release gases and warm air into them and block cold air which causes a lot of problems,” he says.Also read: Cooler For Some, Unliveable For Others: Delhi’s Heat Crisis is a Failure of Law and PolicyAccording to him, temperatures have been rising over the summers and the increasing numbers of houses are a factor. “Earlier the heat used to be lesser,” he tells The Wire. “The number of houses are increasing now, the number of cars are also increasing which have increased the heat.”If we move 20-100 kilometres away from Delhi, the buildings are shorter and this allows for the wind to blow and brings relief, he says. “It has the ground floor and the first floor on top, maximum. Now here, the buildings are 6 or 8 floors, sometimes even 20 or 30 floors where the wind cannot blow. The cars also increase the heat, CNG ones in particular.”‘A thermal commons’There are solutions at various levels to tackle the issue of increasing nighttime temperatures, says Baviskar. Creating a thermal commons would be a short-term immediate emergency response, a public solution because privatised effective cooling is not going to be available, says Baviskar. Repurposing existing facilities – such as metro stations which already have the air conditioning infrastructure in place – could be one way to implement this. “Though as an environmentalist I am a critic of air conditioning because of its huge environmental costs including the fact that the electricity that it depends on is mainly generated from fossil fuels, the fact that air conditioners make it hotter for everybody else, contribute to the urban heat island effect etc…I think the way our temperatures are rising up and up, air conditioning has to be something that is provided on a much wider scale, especially for those times of crisis,” she says.Authorities could also use the idea of cooling stations for the nighttime, she adds. “There are other public buildings which are air conditioned, which you know can be used in a limited way for the hottest nights. So that would be an emergency response.” A possible longer term measure is retrofitting ventilation, and innovating with architects and urban planners to figure out how to create better ventilation in existing informal settlements. The longest-term and hardest thing is to redesign entire neighbourhoods, and put in place building protocols which deal with not just safety standards but also deal with the heat absorbing and radiating capacity of the materials used for construction, she adds.Encouraging passive cooling systems will also make a huge difference, Baviskar notes.“That would in the long term help to institutionalize being able to deal with the worst effects of what is only going to increase – extraordinary heat,” she says.