New Delhi, May 15 -- Households - particularly in low- and middle-income groups are highly prone to night-time heat stress, with indoor temperatures frequently exceeding 32degC, equivalent to 5,700 and 5,800 hours (equivalent to eight months) in the worst impacted households. In most households, it was exceeding this between 3,000 and 5,000 hours of continuous heat, a new study has found. The study, released by the research organisation Climate Trends as part of the India Heat Summit 2026 in Delhi on Wednesday, was carried out across 50 low and middle-income houses in Chennai, between October 2025 and April 2026 using high-resolution sensor data. It found that temperatures peaked between 8 and 9pm at around 35degC and nighttime temperatures (between 8pm - 6am) rarely fell below 31degC even during the cooler months of the study period, flagging the impact is much worse during peak summer months. "The large majority of households recorded between 3,000 and 5,000 hours, translating to four to seven months of persistent exposure," said the study, titled 'Nighttime Thermal Stress in Low and Middle Income Housing in India: Linking Indoor Temperatures and Relative Humidity with Perceptions of Comfort'. "Indoor spaces reached their peak temperatures not at midday but between 8pm and 9pm, as reinforced concrete structures released the heat stored through the day and remained above 34degC well into the night," it said. The study stressed that India's heat governance framework has been built almost entirely around outdoor temperature thresholds and daytime heatwave declarations and there is a need for the integration of indoor heat monitoring into urban Heat Action Plans nationally and to better plan housing infrastructure. The study's most significant policy-relevant finding was that indoor heat exposure is determined by the structural characteristics of housing like construction materials, ventilation, and housing density. Across the 50 households, nearly all had concrete roofs with similar thermal mass properties. "This delayed peak suggests significant heat retention caused by the building materials, which continue to release stored heat well after sunset," said the study. While every high income household in the sample had an air conditioner, every low income household had only a ceiling fan. "The cooling gap is binary: thermal comfort is achievable for those who can afford mechanical cooling. For those who cannot, heat exposure persists through the night," it flags. India currently has over 300 Heat Action Plans, with 100 more in development. None of these mandate indoor temperature monitoring. Aarti Khosla, founder and director, Climate Trends, said the study brings into focus how nighttime heat retention must also receive attention. "That its residents must also face chronic heat exposure indoors, and have it affect their sleep and recovery periods, is a matter that seeks urgent interventions," she said. The Summit consisted of over 200 researchers, scientists and government officials,...