When Nights Turn Deadly
A new study from Climate Resilience for All reveals a hidden climate threat: dangerously hot nights are becoming more frequent and oppressive in cities across the globe.
As country leaders focus on avoiding breaching the +1.5°C threshold of the Paris Agreement, nighttime temperatures in cities are soaring at a rate of as much as 10 times higher than daytime temperatures.
A groundbreaking new report from Climate Resilience for All (CRA), Extreme Heat and the Shrinking Diurnal Range: A Global Evaluation of Oppressive Air Mass Character and Frequency, released in November 2025, reveals that nighttime temperatures are rising at alarming rates in cities across the globe.
Global pattern, local consequences:
CRA’s analysis fills a significant gap in climate research by conducting a first-of-its-kind global examination of the hottest, most oppressive air masses—those that drive deadly heat. It found that cities are not only experiencing on average an extra day of extreme heat every six years, but also—even more alarmingly—nighttime temperatures are rising much more rapidly than daytime highs in a majority of the cities worldwide.
Women are particularly vulnerable to nighttime heat. Physiological factors make them more sensitive to higher temperatures, and societal norms and inequalities amplify their risk. Often shouldering a greater burden of care for older relatives and the sick, it’s usually a woman who stays awake when their family member cannot sleep, sacrificing her own rest and recovery, to care for them. And gender norms, or the threat of violence can keep women from sleeping outside or with windows open, making thermal relief even harder to find.
Now—having analyzed data from 100 cities over the course of three decades—this pivotal study is raising the alarm: we have been in the dark about just how great our risk to these lethal conditions is.
“We want this analysis to mobilize city and health leaders to urgently broaden their view of what is a 24-hour heat crisis,” said Kathy Baughman McLeod, CEO of Climate Resilience for All. This research uncovers a critical blind spot in our understanding of extreme heat.”
The Danger After Dark
When most people think of deadly heat, they picture blistering afternoons under a blazing sun. But the real danger often strikes long after sunset when unrelenting trapped heat of the day intensifies in urban dwellings and prevents essential sleep.
The tossing and turning people experience on a stifling night does more than leave them reaching for caffeine the next day. When a person is not able to recover—especially after extreme daytime heat—it triggers a cascade of negative health impacts, including cardiovascular strain, heat exhaustion and stroke, dehydration, renal failure, compromised immune function, reduced cognitive performance, and even death. Research in fact suggests that the mortality risk on days with hot nights could be as much as 50% higher than on cooler nights.
“Before this analysis, we did not know how rapidly nighttime heat has been rising within the most dangerous air masses,” said Dr. Larry Kalkstein, the lead for Climate Resilience for All’s meteorological team. “It is critical for us to understand how the heat of summer—that sends people to the emergency room—is shifting, and what we are overlooking when we talk about it.”
What is an air mass? An air mass is a large pocket of air with consistent temperature and humidity that moves across regions, shaping local weather. Some air masses bring relief; others bring danger.
In Mumbai, for example, in just under 15 years, the minimum daily temperature during a moist tropical air mass will rise 1°C. Nights during a dry tropical air mass are rising even more rapidly: 1°C in a little over 8 years. For comparison, average global temperatures are currently rising at a rate of 1°C every fifty years.
The pace of nighttime warming is not equal, and many of the fastest-warming cities are also the most vulnerable. Across Africa, eight out of nine cities are seeing nights grow hotter year after year during a moist tropical air mass. In Central and South America, the number climbs even higher—ten out of eleven cities are losing the nightly cooldown their residents rely on.
The same pattern is unfolding across Asia, the Middle East, and North America. Europe, by contrast, has been mostly spared, for now, only because this type of air mass is less common there. But for cities already struggling with limited access to air conditioning, healthcare infrastructure, and other adaptive resources, the danger is mounting fast.
The graphics below show the ten cities with the largest decreases in the difference between daytime temperatures and nighttime temperatures for Dry Tropical (left) and Moist Tropical (right) air masses.
What can we do?
When the world no longer cools after sundown, surviving the heat becomes a 24-hour challenge—and cities must rethink what resilience truly means in this round-the-clock climate.
Some solutions can happen rapidly: integrating nighttime temperatures into heat warning systems, extending cooling center hours, and expanding community outreach to protect the most vulnerable. Others will take longer but yield lasting benefits—expanding tree canopies, investing in reflective surfaces and “cool pavements,” and restoring blue infrastructure like ponds and fountains that help cities breathe after dark.
Health systems must also evolve, training workers to recognize nighttime heat illness, integrating cooling into chronic disease care, and extending energy assistance to make safe nighttime cooling accessible to all. And policymakers need to bring the night into climate planning itself—embedding heat resilience into building codes, energy policy, disaster response, and social protection programs.
The findings from this study are a wake-up call: as nights grow hotter, human resilience means preparing for a world where danger never sleeps.