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Climate change is warming the planet’s temperatures during the day, but also at night. According to a scientific analysis published on Thursday, the number of nights exceeding 25°C has already increased considerably for about a third of the world’s population, which is not without consequences for health.
High nighttime temperatures can become dangerous because they prevent the human body from cooling down and recovering from daytime heat.
The World Health Organization (WHO) recommends keeping room temperatures at or below 24°C at night – a limit above which sleep becomes uncomfortable and can impact the health of vulnerable people (babies, the elderly or those with chronic diseases).
The study, conducted by Climate Central, an independent group of scientists and climate communicators, compared average annual warm nights between 2014 and 2023 with a hypothetical world without human-caused climate change, using peer-reviewed methodology and models that incorporate historical data. But the data is patchy and sometimes incomplete in some countries, which is why the researchers chose an alternative world where the only thing that has changed is the amount of carbon in the atmosphere.
It concludes that over the past decade, about 2.4 billion people have experienced at least two extra weeks of nights with temperatures above 25°C compared to a world without climate change.
Trinidad and Tobago in the Caribbean saw the largest average increase of any country, with 47 additional nights per year above 25 degrees Celsius. The Indian city of Mumbai suffered two additional months of hot nights.
“Higher nighttime temperatures, particularly during warmer times of the year, can interfere with sleep and reduce physical recovery from high daytime temperatures, which can have cascading health impacts.“, Nick Obradovich, chief scientist at the US-based Laureate Institute for Brain Research, told AFP.
The 25°C threshold”is not an absolute value below which health is good and above which it deteriorates“, but it has varying consequences depending on the person, especially if the heat is combined with humidity, sometimes creating conditions that can become fatal, adds this scientist who did not participate in the study.
Several studies have shown that nighttime temperatures above 25°C deteriorate the quality and duration of sleep – which is vital for human functioning – and increase the risks of stroke, cardiovascular disorders and mortality, particularly among the elderly and those on low incomes.
Discover all our tips for sleeping better despite the heat.