With the heatwave episode strictly speaking, "We're going to have at least four days where the temperature doesn't drop at night. And so, we're going to start seeing the effects." Agnès Ricard-Hibon, an emergency physician in Val-d'Oise and spokesperson for the Samu Urgences de France union, told AFP on Tuesday. Sunday and Monday, when the thermometer read 35°C in many parts of the country, were already synonymous with nights "very unpleasant" with the mercury not falling below 20 degrees, according to Tristan Amm, forecaster at Météo-France.
The brain, where neurons regulating temperature and sleep are closely connected, is indeed very sensitive to heat, which raises the central thermostat and activates stress systems. When it's very hot, blood vessel dilation in the skin is less efficient, heat loss is less, and falling asleep is more difficult, sleep specialists point out.
“A global threat to sleep”
High nighttime temperatures also increase waking times and reduce deep sleep, which is considered the most restorative.
"While attention is generally focused on maximum daytime temperatures, it is nighttime temperatures that will pose the greatest health risk, particularly for vulnerable populations.", according to the United Nations World Meteorological Organization (WMO). With these tropical nights, "The body cannot recover from continuous high heat. This leads to an increase in heart attacks and deaths," underlines the organization.
Read alsoUnprecedented heatwave in Europe: heat records, fires, health alerts
A review of scientific literature, published in mid-2024 in the journal Sleep Medicine, also stressed that "Rising temperatures induced by climate change and urbanization pose a global threat to sleep." However, excessive sleep deprivation compared to needs disrupts the body's recovery.
In the short term, the main documented effects are cognitive: drowsiness, fatigue, risk of work or traffic accidents. In the long term, this can lead to " debt " harmful due to its action on metabolism.
Cool shower and limit alcohol and coffee
Resistance to stress can also decrease, and the risk of relapse or the anchoring of a mental illness can increase. To sleep better in high heat, specialists recommend strengthening the mechanisms that cause temperature fluctuations during day/night cycles and paying attention to “enemies of sleep.”
Read alsoHeatwave: Why does an apartment at 25°C seem comfortable in winter and stifling in summer?
Before bedtime, for example, it is recommended to take a cool but not ice-cold shower, to reduce stimulants, such as coffee, or to limit alcohol, which promotes sleep but slightly increases internal temperature. To partially recover from sleep deprivation, napping has proven its benefits.
English 
French