The heat-stress season is getting longer. And heatwaves don’t just affect us during the day. Researchers are increasingly warning about the consequences of hotter nights for sleep and health.
As temperatures rise, so does the risk of a bad night’s sleep. While much of the focus during heatwaves is on staying safe when temperatures soar above 35°C, the effects of hot nights on sleep and health are less well understood.
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A recent study using data from the Copernicus Climate Change Service found that nights are warming particularly quickly. Since the 1970s, the minimum “feels-like” temperature during the 10 warmest nights of the year has risen by 0.32°C per decade globally, according to the Copernicus study. The measure, known as the Universal Thermal Climate Index (UTCI), takes temperature, humidity, wind and radiation into account.
The number of tropical nights, when temperatures do not fall below 20°C, has also increased by around two nights per decade.
Nighttime is when the body is supposed to recover
The researchers say the trend matters because hot nights give the body less opportunity to recover from heat experienced during the day. Hot days followed by hot nights are also becoming more common: in Europe, these so-called ‘compound heat events’ have increased by 73% since the 1970s.
During sleep, heart rate and physiological activity normally fall, while the body’s core temperature drops. This helps the nervous, immune and cardiovascular systems recover from the demands of the day.
But when a bedroom remains too hot, the body has a harder time getting rid of heat. Blood vessels near the skin dilate to release warmth and the heart has to work harder to circulate blood. Sweating can also cause dehydration.
The result can be more difficulty falling asleep, more frequent awakenings and less restorative sleep.
The problem can become particularly serious when hot nights continue 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 rise again.
Researchers are increasingly interested in whether this repeated lack of overnight recovery could help explain why prolonged heatwaves pose such a significant health risk.
What you can do to stay cool during the night
Most homes in areas that have not been used to the heat are not adapted to rising temperatures and may struggle to cool down after sunset. As a result, bedrooms can remain hot and humid long after the worst of the daytime heat has passed.
The World Health Organisation (WHO) recommends keeping indoor temperatures below 24°C at night, where possible, to help the body cool down and support better sleep during heatwaves.
Here are seven measures you can take to cool down your home for a better night’s sleep during heatwaves:
- Open windows after dark if the air outside is cooler than indoors.
- Close blinds, shutters or curtains during the day to prevent the room heating up.
- Use light bed linen and loose, lightweight clothing.
- Take a cool shower or bath before bed.
- Drink water regularly, but avoid too much alcohol and caffeine.
- Move to the coolest room in the home if your bedroom becomes too hot.
- Turn off unnecessary electrical appliances, which generate additional heat.
