Too hot to sleep? How heatwaves at night affect our health

Global warming isn’t just about heat during the day. Nights are also getting hotter^1–3. 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.

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.

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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.

Night-time heat can also strain the body. The heart pumps more blood to the skin to shed internal heat, which elevates the heart rate. Sustained sweating can lead to dehydration. These physical demands can be particularly taxing for older adults and people with weak cardiovascular systems.

In hot weather, all these disruptions combine to increase health risks. Heat exposure increases physiological arousal, as reflected in an elevated heart rate and increased sweating. Waking up to drink water or change position interrupts the restorative sequence of sleep stages. Broken sleep can in turn weaken cardiovascular, metabolic, immune, cognitive and emotional recovery, contributing to fatigue, impaired attention and poor regulation of physiological processes the next day^5,7,8.

Medical researchers understand the parts of these processes. But they haven’t quantified them adequately and fully determined how they’re linked in a vicious circle. Climate change means that many more people will be exposed to hot nights more often. As we all adapt, researchers and policymakers must ask not just how hot the day will be, but whether the night will allow people to cool down, sleep and recover (see ‘Night-time heat: five research priorities’).

Heat health risks need to be tracked

Conventional night-time heat metrics based on meteorological anomalies are useful for tracking climate trends. For example, a tropical night is often defined as one in which the outdoor air temperature stays above 20 °C — or higher in some regions, such as 25 °C in South Korea. Indices of consecutive hot nights count runs of two or more nights exceeding a temperature threshold¹.

But these data are inadequate for medical purposes. They fail to capture the temperature indoors and the exact timing and duration of heat exposure paired with sleep quality through the night, for instance. We argue that researchers and policymakers should instead assess night-time risk in terms of an imbalance between the thermal load someone experiences and their body’s capacity to recover.

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Thermal load can be quantified as the number of hours during which bedroom conditions exceed a health-relevant temperature threshold during the sleeping period, and participants’ recovery can be measured in several ways.

Cost-effective sensors can be used to record air temperature and humidity every few minutes^8,9. In field studies, these sensors could be placed in bedrooms, and participants could wear devices on the wrist that record sleep timing, sleep continuity, movement, skin temperature, heart rate and heart-rate variability^8,9. Measuring morning blood pressure, fatigue, sleepiness and attention can indicate whether recovery was completed overnight.

Small samples could be used to validate larger studies through detailed monitoring of body core temperature and sleep stages, using sensors and electrodes applied to the skin^5,8. With informed consent, participants’ records could be linked to their health-care data.

Such methods are already technically feasible and have been used to investigate the effects of air quality on sleep¹⁰ — uncovering an association between warmer, poorly ventilated bedrooms and poorer sleep, particularly among older adults^8,9. A platform called HeatSuite has demonstrated the feasibility of integrating data from environmental sensors with those from wearable devices, such as blood-pressure and temperature measurements, in older adults¹¹.

These approaches should now be extended to cohorts monitored before, during and after hot nights. Repeated observations are essential to test whether incomplete overnight recovery leaves residual physiological strain for days. Environmental, sleep and physiological measures should be collected for several weeks, supplemented by short daily surveys of symptoms, medication and cooling behaviours.

Such studies could establish the degree to which incomplete recovery contributes to increased mortality and hospital admissions during hot nights^2,3. They could explore the effects of long or repeated moderately hot nights, and test whether real-time indoor temperature is more relevant than is an outdoor minimum.