“There is an increasing increase in international publications on sleep and temperature, the last in 2024 on sleep hygiene in the face of heat,” Dr. Fabien Sauvet, a researcher at Paris Cité, told AFP.
The brain, where neurons regulating temperature and sleep are very connected, is very sensitive to heat. The strong heat increases the central and active thermostat of stress systems.
Because of the increase in temperatures, humans would have lost an average of 44 hours of sleep on average in the first two decades of the 21st century compared to previous periods, according to a study published in 2022 in the journal One Earth.
The accentuation of global warming could lead to up to 50 to 58 hours of sleep lost per person and per year by 2099, according to this work managed by Kelton Minor (Copenhagen University), based on data of more than 47,000 carlets of connected bracelets in 68 countries, crossed with meteorological data.
“The rise in temperatures induced by climate change and urbanization is a planetary threat to sleep” has confirmed a review of the scientific literature published in mid-2014 in Sleep Medicine. “Other studies and experiences are now urgently necessary to promote adaptation,” said its authors, including Kelton Minor.
To fall asleep and sleep as well as possible, one of the conditions is to lower your internal thermostat.
“Physiological acclimatization to heat has a cost for the body: we sweat more, and faster, for example, but you have to hydrate more. And it has limits, so that in periods of heat wave, the main thing is to modify its behavior (activities, rhythms, clothes …)”, noted Dr Sabt.
However, we can “tolerate higher temperatures than we commonly think”, according to him.
“A number of laboratory studies, as with sportsmen before the Paris 2024 Olympic Games, show that a good sleep quality is possible when the room temperature, well airy, is between 24 and 26 degrees Celsius, up to 28 degrees, if you sleep in shorts and t-shirts with just a sheet”, detailed Dr Sauvet, refuting “the false belief that the room must be 18-20 degrees”.
And “if we sleep all the time with air conditioning, we will never acclimatize,” he warned.
Fight the “sleep enemies”
If “up to around 28 degrees, we can sleep well, beyond that becomes much more complicated,” Armelle Rancillac, researcher in neuroscience in France, told AFP at AFP at the Collège de France.
However, an excessive sleep deprivation in relation to the needs disrupts the recovery of the body.
In the short term, the main documented effects are cognitive: drowsiness, fatigue, risk of accidents at work or traffic. In the long term, this can cause a harmful “debt”, not only for the most vulnerable (seniors, young children, chronic patients …).
“It plays on our metabolism, promoting weight gain, the development of diabetes, cardiovascular diseases, even neurodegenerative diseases like Alzheimer’s,” summed up neuroscientist.
This debt can also decrease stress resistance and increase the risk of relapse or anchoring a mental pathology.
To sleep better by strong heat, the objective is to “strengthen the mechanisms fluctuating our temperature during day/night cycles and eliminate, or at least pay attention, to sleep enemies”, according to Armelle Rancillac.
Before bedtime, it is advisable to take a fresh but not icy shower, to reduce exciting, such as coffee, to limit alcohol, which promotes falling asleep but slightly increases the internal temperature.
And “after a sports session, especially no jacuzzi, but rather expose yourself to the outside temperature or take cold baths”, advised Dr Sauvet.
To limit the impact of the lack of night sleep, the benefits of the nap, during the hottest hours of the day, were proven.
In order not to alter the following night, the ideal is “a short nap, from 30 to 40 minutes, and before 2 pm”, according to the researcher.
And why not in a hammock, which, “unlike a foam mattress, does not accumulate body heat over the hours”, or in a sleep box, as there are already in some hospitals for staff.