Hot nights: a challenge for sleep and a threat to health

With the heatwave episode in the strict sense , "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 doctor in Val-d'Oise and spokesperson for the Samu Urgences de France union, told AFP on Tuesday.

Already Sunday and Monday, when the thermometer showed 35°C in many parts of the country, were synonymous with "very unpleasant" nights 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 in fact very sensitive to heat, which raises the central thermostat and activates stress systems.

When it is very hot, the dilation of blood vessels in the skin is less efficient, heat loss is less, and falling asleep is more difficult, sleep specialists point out.

High nighttime temperatures also increase waking times and reduce deep sleep, which is considered the most restorative.

A harmful debt

"While the focus is usually on daytime highs, it is nighttime temperatures that pose the greatest health risks, especially for vulnerable populations," according to the United Nations World Meteorological Organization (WMO). With these tropical nights, "the body cannot recover from sustained high heat. This leads to an increase in heart attacks and deaths," the organization emphasizes.

A review of the scientific literature, published in mid-2024 in Sleep Medicine, also highlighted 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, and the risk of workplace or traffic accidents. In the long term, this can create a harmful "debt" through its effect on metabolism.

Resistance to stress can also decrease, and the risk of relapse or anchoring of a mental illness can increase.

To sleep better in extreme heat, experts recommend strengthening the mechanisms that cause temperature fluctuations during day/night cycles and keeping an eye out for the "enemies of sleep."

Before going to bed, for example, it is advisable to take a cool but not ice-cold shower, to reduce stimulants, such as coffee, or to limit alcohol, which helps you fall asleep but slightly increases your internal temperature.

To partially recover from sleep deprivation, napping has proven its benefits.