The human brain is very sensitive to heat, with higher temperatures raising the body's central thermostat and activating stress systems.
Scientists are increasingly exploring mechanisms that can help the body adapt to rising temperatures affecting our sleep and leading to health complications.
"Rising temperatures induced by climate change and urbanisation pose a planetary threat to human sleep, and therefore health, performance, and wellbeing," according to a 2024 review of scientific literature published in the journal Sleep Medicine.
Humans lost an average of 44 hours of sleep a year during the first two decades of the 21st century compared to earlier periods, according to a 2022 study published in the journal One Earth, which linked the data to rising temperatures.
The intensification of global warming could lead to an annual loss of 50 to 58 hours of sleep per person by 2099, according to research led by Kelton Minor, from the University of Copenhagen, based on data gathered from more than 47,000 individuals in 68 countries.
"Interventional studies and field experiments are now urgently needed to foster adaptation and safeguard the essential restorative role of sleep in a hotter world," Minor and other authors of the paper said.
Neurons regulating temperature and sleep in the brain are highly interconnected, and lowering the body's internal thermostat is key to improving sleep quality.
Adapting to heat comes at a cost to the body, according to Fabien Sauvet, a researcher at Paris Cite University.
"We sweat more and faster, for example, but it requires additional hydration. And it has limits, so during heatwaves, the most important thing is to adapt our behaviour," such as activities, schedules and clothing, Sauvet said.
But humans could "tolerate higher temperatures than commonly thought", he added, pointing to several studies showing that good sleep quality can be achieved with a room temperature of up to 28 degrees Celsius (82.4 degrees Fahrenheit).
Challenging "the false belief that the bedroom must be at 18-20C", he said that sleeping in light clothing such as a t-shirt and shorts, and with a simple sheet as well as good ventilation, could help dealing with a few more degrees.
"If we always sleep with air conditioning, we will never acclimatise," he said.
- Fight 'sleep enemies' -
Armelle Rancillac, a neuroscientist at the Centre for Interdisciplinary Research in Biology, said anything beyond 28C "becomes much more complicated".
An excessive lack of sleep is known to disrupt the body's recovery.
In the short term, this can lead to drowsiness, fatigue and a higher risk of accidents at the workplace or on the road.
In the long run, it can create a harmful sleep "debt", impacting our metabolism and increasing the risk of weight gain, diabetes, cardiovascular diseases and even neurodegenerative diseases like Alzheimer's, Rancillac said.
A sleep deficit can also reduce stress resistance and have a negative impact on mental health.
To sleep better in a hotter environment, Rancillac stressed that there is a need to "eliminate or at least pay attention to sleep enemies".
Before bedtime, it is recommended to take a cool shower -- but not an icy one -- reduce stimulants like coffee, and limit alcohol, which facilitates falling asleep but slightly raises the internal body temperature.
Avoid hot tubs after a workout, opting instead for outdoor temperatures or a cold bath, said Sauvet.
Napping during the hottest hours of the day have also been proven to mitigate the impacts of a sleep deficit.
Short naps -- "30 to 40 minutes, and before 2:00 pm" -- are ideal, so as not to interfere with a good night's sleep, according to the researcher.
Urban temps turning cities into 'ovens,' UN Chief Heat Officer warns
Washington (AFP) May 13, 2025 -
Whether in Miami, Athens, or Santiago, dedicated ambassadors are stepping up to tackle extreme urban heat around the world.
Eleni Myrivili, one of the field's pioneers who currently serves as the Global Chief Heat Officer for UN-Habitat, spoke to AFP about the urgent need to redesign cities to keep asphalt-riddled areas from turning into impossible-to-escape "ovens" for the most vulnerable populations.
Why are cities at the center of your work?
We identified that cities are basically the Ground Zero of heat, where we have the most dire impacts.
Cities today are heat traps and they are built for other types of temperatures, for a different climate. So we need to understand and totally change our perspective as to how we retrofit and develop new areas.
We do it in ways that take into account the fact that we will be dealing with a totally different climate in the next decades.
Can you give us examples of solutions your team worked on?
In Athens, we worked on the categorization of extreme heat, so that there are specific thresholds that trigger different types of policies and actions during heatwaves to make sure that we protect the most vulnerable populations.
We created heat campaigns, so people understand how dangerous heat can be for their health and what they should be doing during heatwaves.
Creating shading structures specifically for people waiting for trains or waiting for buses, so that these have special cooling aspects, like misters or like white or green roofs on them so they do not absorb heat while people are standing right under them. Of course, almost all of us have created plans for nature-based solutions and for bringing more nature into the cities.
How has climate change impacted your region?
On average in the Mediterranean part of Europe, we have about 29 days of strong heat stress (relative to the average for the 1991-2020 reference period), but we jumped from the 29 (average) to 66 (days) in the summer of 2024.
That's what we mean when we say that the average global temperatures have surpassed 1.5 degrees Celsius from the pre-industrial era, it means that on the ground we see these extraordinary heat seasons.
How can cities prepare against these new norms?
We need to be prioritizing shade, wind and water, and, of course, nature.
This also means that we have to bring within our development and city planning projects other types of expertise. We have to bring in landscape architects. We have to bring in ecologists, foresters, people who understand thermodynamics.
On a very large scale, but also on a very local scale, we have to consider water as the most crucial element that will break us or make us as we deal with rising heat.
In contrast, can you give us an example of what maladaptation can look like in urban spaces?
Air conditioning is a great example of maladaptation because it creates more problems than it solves.
Air conditioning is extremely important to the most vulnerable populations, we have to make sure they have access to air conditioning. But we have to understand that air conditioning has to be used carefully, and not as a panacea that is just going to help us deal with extreme heat.
We can't air condition ourselves out of this mess that we've created, because air conditioners are an extremely selfish way of dealing with extreme heat. You cool your own little space, while at the same time, you're blowing more hot air into the public spaces.
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