For decades, Mumbai has been known as the city that never sleeps. Increasingly, however, it is also becoming a city that never cools down. A new study published in the Journal of Urban Management by Harekrishna Manna et al. has found that nighttime temperatures across the Mumbai Metropolitan Region (MMR) have risen steadily over the past 25 years, with some densely urbanised parts of the city experiencing temperatures more than 5°C higher than surrounding areas. The research warns that Mumbai’s heat challenge is no longer merely a daytime phenomenon. The city is now accumulating and retaining heat after sunset, creating what scientists call a nighttime Urban Heat Island (UHI) effect—a condition with profound implications for public health, energy consumption, urban planning and climate resilience. The study arrives at a critical moment. Indian cities are experiencing more frequent and intense heatwaves, while urban populations continue to expand rapidly. As climate change raises baseline temperatures, the combination of global warming and local urban heat is creating a dangerous double burden for millions of residents. The Mumbai findings offer not only a warning but also a roadmap for cities across India and the Global South. Heat is usually associated with the hottest part of the afternoon. But nighttime temperatures can be equally important because the body needs cooler conditions after daytime exposure to heat. When temperatures remain high after sunset, people have less opportunity to recover from heat stress. This can increase the risk of heat exhaustion, cardiovascular illness and sleep disruption, particularly among older people, children and those living in densely built and poorly ventilated neighbourhoods. The researchers analysed nighttime land surface temperatures across Mumbai from 2000 to 2025 using satellite observations and spatial modelling. The analysis found persistent nighttime warming across built-up areas, particularly in southern and central Mumbai. The urban