As climate change pushes daytime temperatures to new extremes, a quieter but equally troubling trend is emerging: nights are warming even faster. While much of the public health advice during heatwaves focuses on staying cool when the mercury tops 35°C, researchers are increasingly concerned about what happens after sunset. A growing body of evidence suggests that hot nights are not just uncomfortable—they are a serious health risk in their own right.
According to a recent analysis based on data from the Copernicus Climate Change Service, the minimum “feels-like” temperature during the ten warmest nights of the year has risen by 0.32°C per decade globally since the 1970s. This measure, known as the Universal Thermal Climate Index (UTCI), accounts for temperature, humidity, wind, and radiation. In Europe, the number of tropical nights—when the temperature does not drop below 20°C—has increased by roughly two nights per decade.
Why hot nights matter for health
Nighttime is when the body is supposed to recover. During sleep, heart rate and physiological activity slow down, and the body’s core temperature drops, allowing the nervous, immune, and cardiovascular systems to repair the wear and tear of the day. But when a bedroom remains too warm, the body struggles to shed heat. Blood vessels near the skin dilate to release warmth, forcing the heart to work harder, while sweating can lead to dehydration.
The result is a cascade of sleep disturbances: difficulty falling asleep, more frequent awakenings, and less restorative sleep. Over a single night, this can leave you groggy and irritable. But the real danger emerges when hot nights persist for several days in a row. After a night of disrupted sleep and incomplete cooling, the body starts the next day already under strain—just as temperatures begin to climb again. This repeated lack of overnight recovery may help explain why prolonged heatwaves pose such a significant health risk, particularly for vulnerable populations like the elderly and those with pre-existing conditions.
Researchers are now paying closer attention to these “compound heat events,” where hot days are followed by hot nights. In Europe, the frequency of such events has increased by 73% since the 1970s, according to the Copernicus study. This trend is especially pronounced in southern and central Europe, where cities like Athens, Madrid, and Rome are experiencing more tropical nights than ever before.
Adapting homes and habits
Many European homes, particularly in regions that historically did not need to cope with intense heat, are poorly designed for these new conditions. Buildings that absorb heat during the day can remain stuffy and humid long after sunset, making it difficult for residents to cool down. The World Health Organisation (WHO) recommends keeping indoor temperatures below 24°C at night, where possible, to support the body’s natural cooling and promote better sleep during heatwaves.
There are practical steps you can take to keep your home cooler at night. Opening windows after dark, when the outside air is cooler than indoors, can help. Closing blinds, shutters, or curtains during the day prevents rooms from heating up in the first place. Using light bed linen and loose, lightweight clothing can also make a difference. A cool shower or bath before bed can lower your core temperature, and staying hydrated—while avoiding excessive alcohol and caffeine—helps your body regulate heat. If your bedroom becomes too hot, consider moving to the coolest room in the house.
These measures are especially important for those living in urban areas, where the “heat island” effect can make nights even warmer. Cities like Paris, Berlin, and Warsaw are increasingly investing in green spaces and reflective surfaces to mitigate this, but individual actions remain crucial.
As heatwaves become more frequent and intense across Europe, understanding the health impacts of hot nights is vital. The issue is not just about comfort—it is about protecting public health in a warming world. For more on how heatwaves are straining resources across the continent, see our coverage of Western Europe's repeated heatwaves. And for those who struggle to sleep in unfamiliar places, our guide to overcoming the first-night effect offers useful tips.
