GoKawiilGlobal warming isn’t just about heat during the day. Nights are also getting hotter1–3. In May, Delhi recorded an overnight minimum temperature of 32.4 °C, and other parts of India remained 3–5 °C above normal after dark. As Western Europe experienced its hottest June on record, France registered a record overnight low of 22 °C, and East Saxony, Germany, sweltered at a historic 29.4 °C. In July, record night-time warmth exceeding 27 °C added to the mugginess of cities on the US Gulf and Atlantic coasts.
Heat affects people’s sleep (see ‘Global sleep deficit’). Two-thirds of respondents to a UK survey said they struggled to sleep during the June heatwave, with almost half losing at least three hours of sleep each night (see go.nature.com/5rcv6y). An analysis of more than 1,300 cities found that, during 2020–25, each resident lost nearly 56 hours of sleep, on average, each year owing to night-time heat4.
The consequences go beyond lost sleep. Across 28 East Asian cities, intense heatwaves at night were associated with a 3% increase in deaths (excluding those from injuries and other external causes)3. In Hong Kong, long, hot nights — around 30 °C for 12 hours — were linked to a 3.1% rise in emergency hospital admissions, reaching 5.3% among older and poorer people2.
But scientists don’t fully understand how hot nights affect health and sleep. To get the full picture, measuring temperatures alone is insufficient; people’s physiological responses to heat must also be studied. Here’s what is needed to better understand the link and manage night-time heat.
Source: Ref. 4
At night, people are biologically programmed for recovery
Sleep is crucial for health because it enables our bodies and brains to recover from daily strains. Regulated by our circadian system, our levels of physiological arousal and cardiovascular activity fall when we sleep. Our nervous system, metabolism and immune system are rebalanced. Different stages of sleep help us to consolidate memories and support mechanisms of cognitive and emotional control5.
To save lives in heatwaves, focus on how human bodies work
As we wind down and prepare for sleep, our core body temperature drops, initiating restorative processes. Peripheral blood vessels dilate and transfer heat from the centre of the body to the skin and extremities. But when the temperature in the bedroom is too high, and especially when it is humid, the body’s cooling is impaired — and sleep quality suffers5.
When people are awake, they can drink water, move around, open windows, use fans or seek cooler spaces to help restore thermal balance. When they are asleep, however, their ability to take actions to control body temperature is limited. Thus, it is harder to get to sleep and stay asleep in a hot room6.
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