France experiences rising frequency of tropical nights

France is experiencing a significant increase in ‘tropical nights,’ defined as periods where temperatures do not drop below 20°C between sunset and sunrise. Since the 1950s, the frequency of these nights has shifted from rare occurrences, once limited to the Mediterranean coast, to a regular summer phenomenon affecting northern regions like Brittany.

These rising nocturnal temperatures interfere with sleep, as the human body requires a slight internal cooling to enter deep sleep. The phenomenon is exacerbated by thermal inertia in buildings, where walls and roofs release stored daytime heat at night, and by the urban heat island effect in cities. To mitigate discomfort, experts suggest practical measures such as closing shutters and curtains during the day to block solar heat and creating cross-ventilation by opening opposing windows in the evening.