Climate change is leading to a rise in night-time warming, which is impacting sleep quality and human health in India and across the globe. Night-time temperatures have increased even more rapidly than daytime temperatures as the world heats up due to climate change, primarily caused by burning fossil fuels like coal, oil, and gas.

Roxy Mathew Koll, Climate Scientist, Indian Institute of Tropical Meteorology, Pune, says, “The urban heat island effect is most visible in the night-time temperatures. Cities turn into urban heat islands when buildings, roads, and other infrastructure absorb and re-emit heat, causing cities to be several degrees hotter than surrounding rural areas. During the day, the sun’s rays reach as shortwave radiation and heat the Earth’s surface. At night, the heat escapes as longwave radiation. While shortwave radiation can easily penetrate through and reach the surface, the longwave gets trapped easily by concrete and clouds. The high-rise buildings and concrete setups in the cities do not let the excess heat escape during the night. As the temperatures do not cool down, the heatwave continues into the night. Open green spaces and natural environments with trees can help release the heat faster during the night.”

Warming nights in India: An alarming trend 

Climate change has had a major influence on night-time temperatures above 25 degrees Celsius in India. The new analysis shows that approximately 50 to 80 days each year were added above this threshold by climate change in cities across Kerala, Karnataka, Maharashtra, Tamil Nadu, Punjab, Jammu and Kashmir, and Andhra Pradesh between 2018 and 2023. According to Mahesh Palawat, Vice President – of Meteorology and Climate Change, Skymet Weather, “In the absence of pre-monsoon rains, India has been battling prolonged heatwave conditions. Usually, due to controlled mechanisms of nature, we do see some kind of weather activity after a four to five-day spell of a heatwave. However, due to erratic weather patterns, unabated hot winds from Sindh and Rajasthan regions with speeds between 15-20 kmph, led to the constant rise in day as well as night temperatures. This led to continuous insulation during the night time, interrupting the night time cooling.” Among the metro cities, Mumbai has seen the biggest changes in the night-time temperatures, with the city experiencing an additional 65 days of warmer nights due to global warming.

West Bengal and Assam are the regions that have been most impacted, with cities like Jalpaiguri, Guwahati, Silchar, Dibrugarh and Siliguri experiencing between 80 and 86 additional days each year above the 25 degrees Celsius threshold due to climate change, on average.

Several cities across India saw between 15 and 50 additional days where the minimum temperatures exceeded 25 degrees Celsius due to the influence of climate change, including Jaipur, with an additional 19 hot nights attributable to climate change.

Meanwhile, in both observations and in the counterfactual climate, the night-time summer temperatures across India often exceed 20 degrees Celsius over the entire summer period. The cities that had the largest number of days where the minimum temperature exceeded 20 degrees Celsius due to climate change are Gangtok, Darjeeling, Shimla, and Mysore, with an average of 54, 31, 30, and 26 days added by climate change, respectively.

Higher night-time temperatures can cause physiological discomfort and impact human health by preventing body temperature from cooling off during the night, increasing mortality risks. There is also a growing body of evidence that as night-time temperatures rise, it is adversely affecting the quality and length of sleep. Poor sleep adversely affects physical and mental health, cognitive functioning and even life expectancy. Hot nights can have disproportionate impacts on vulnerable groups, including the elderly and people who do not have access to proper cooling mechanisms.

Prof Anjal Prakash – Clinical Associate Professor (Research) and Research Director at Bharti Institute of Public Policy, Indian School of Business, says, “The rise in night-time temperatures due to climate change poses a severe threat to the vulnerable populations in India, especially in metropolitan areas like Mumbai. As global warming adds an additional 50 to 80 warm nights annually in states such as Kerala, Karnataka, Maharashtra, Tamil Nadu, Punjab, Jammu and Kashmir, and Andhra Pradesh, the impact on those without access to proper cooling mechanisms is profound. High night-time temperatures limit nocturnal cooling, exacerbating health issues such as heat strokes and cardiovascular diseases. The inability to cool down at night can lead to chronic sleep deprivation, further weakening immune systems and increasing susceptibility to illness. For marginalized communities who already face hardships, this added climate stress can be devastating. It highlights the urgent need for climate resilience initiatives, equitable access to cooling solutions, and comprehensive public health strategies to mitigate the adverse effects of a warming world in India.”

New records set for night-time heat in India: 

These findings come during a week that saw new records for night-time heat in several Indian cities. On June 19, Delhi shattered the all-time high minimum temperature record, with the mercury reaching 35.2 degrees Celsius overnight. Delhi recorded almost four additional nights over 25 degrees Celsius due to climate change between 2018 and 2023, according to the Climate Central analysis.

On 18 June, Alwar in Rajasthan had a minimum temperature of 37 degrees Celsius, the highest ever night-time temperature since records began in 1969. Alwar experienced almost nine additional nights over 25 degrees Celsius that are attributable to climate change between 2018 and 2023. In Uttar Pradesh, Lakhimpur Khiri, Shahjahanpur and Varanasi also witnessed their highest recorded minimum temperatures at 33 degrees Celsius, 33 degrees Celsius and 33.6 degrees Celsius, respectively, this week. Varanasi saw four additional nights over 25 degrees Celsius due to climate change from 2018 to 2023. These increasingly frequent extreme night-time temperatures are contributing to heat stress, exhaustion and heat-related deaths.

The average person on Earth experienced 4.8 additional days above 20 degrees Celsius as a result of higher night-time temperatures induced by climate change between 2018 and 2023. Meanwhile, over this time period, the average person experienced an additional 11.5 days per year above 25 degrees Celsius.

Michelle Young, Climate Impacts Research Associate at Climate Central, observes, “Hot nights prevent people from recovering from extreme heat during the day and are likely to have shortened and disrupted people’s sleep, with a range of serious knock-on effects on physical and mental health. We also know these impacts are not being experienced evenly or equally, with disparities between and within countries based on income, access to air-conditioning, age and other factors. As one of the countries worst affected by extreme heat caused by climate change, higher minimum temperatures overnight have made this year’s record-breaking heatwave in India even more deadly. As night-time temperatures continue to shoot up, there will be more and more of these sleepless nights until the world stops burning fossil fuels like coal, oil and gas and cutting down forests.”