Nighttime Heatwaves and Sleep: How Hot Nights Affect Your Health

Climate change is making nights hotter, not just days. Extreme nighttime temperatures are becoming more common worldwide, reducing sleep quality and increasing health risks^1–3. In May, Delhi recorded a nighttime low of 32.4 degrees Celsius, while temperatures in other parts of India remained 3 to 5 degrees Celsius above average after sunset. Western Europe also experienced its hottest June on record. France recorded an overnight low of 22°C, while East Saxony in Germany reached a historic nighttime temperature of 29.4°C. In July, nighttime temperatures above 27°C intensified muggy conditions in cities along the U.S. Gulf and Atlantic coasts.

Hot nights can seriously disrupt sleep. Two-thirds of respondents in a UK survey said they struggled to sleep during the June heatwave, and almost half lost at least three hours of sleep each night (go.nature.com/5rcv6y). An analysis of more than 1,300 cities found that between 2020 and 2025, residents lost an average of nearly 56 hours of sleep each year because of nighttime heat⁴.

The health effects extend beyond sleep deprivation. Severe nighttime heatwaves were associated with a 3% increase in deaths across 28 cities in East Asia, excluding deaths from injuries and other external causes³. In Hong Kong, prolonged hot nights of around 30°C lasting 12 hours were linked to a 3.1% increase in emergency hospital admissions. The increase reached 5.3% among older adults and people with lower incomes².

Scientists still do not fully understand how nighttime heat affects sleep and health. Outdoor temperature alone does not provide the complete picture. Researchers must also examine indoor conditions, humidity, individual physiology, sleep quality and the body’s ability to recover overnight. Understanding this relationship is essential for managing the growing health risks of hot nights.

The human body needs cool nights to recover

Sleep is essential for physical and mental health because it allows the body and brain to recover from daily stress. Controlled by the circadian system, physiological arousal and cardiovascular activity normally decline during sleep. The nervous, metabolic and immune systems are rebalanced, while different sleep stages support memory consolidation and emotional and cognitive regulation⁵.

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As people prepare for sleep, core body temperature naturally falls. Blood vessels near the skin dilate, moving heat from the body’s core to the skin and extremities. However, when a bedroom is too hot or humid, this cooling process becomes less effective, reducing sleep quality⁵.

During the day, people can help restore thermal balance by drinking water, opening windows, using a fan, moving to a cooler location or changing their clothing. During sleep, however, the ability to respond to heat is limited. As a result, it becomes harder to fall asleep, remain asleep and achieve restorative sleep in a hot bedroom⁶.

Nighttime heat also places additional strain on the body. The heart must pump more blood toward the skin to release internal heat, increasing heart rate. Persistent sweating can cause dehydration, while the cardiovascular system continues working harder instead of recovering. These demands can be especially dangerous for older adults and people with cardiovascular conditions.

When these effects occur together, heat exposure can significantly increase health risks. Higher temperatures increase physiological arousal, heart rate and sweating. Waking to drink water or change position interrupts the normal progression of sleep stages. Repeated sleep disruption can impair cardiovascular, metabolic, immune, cognitive and emotional recovery, resulting in fatigue, reduced alertness and altered physiological function the following day^5,7,8.

Researchers understand some of these biological processes, but they have not yet been fully quantified or connected into a clear model of nighttime heat stress. Climate change is exposing more people to hot nights more frequently. Adaptation strategies must therefore consider more than daytime temperatures. Researchers and policymakers also need to determine whether people can cool down, sleep and recover safely overnight.

Nighttime heat risks need better health monitoring

Traditional nighttime heat indicators based on weather anomalies are useful for tracking climate trends. A tropical night, for example, is often defined as a night when outdoor temperatures remain above 20°C. In some regions, including South Korea, the threshold is higher at 25°C. The Consecutive Heat Index measures the number of consecutive days with at least two nights above a defined temperature threshold¹.

However, these indicators are not sufficient for assessing medical risk. They do not show exactly when or how long people are exposed to heat, how hot their bedrooms become, how humidity affects them or whether they achieve restorative sleep. Nighttime heat risk should instead be evaluated by comparing the total heat load a person experiences with the body’s capacity to recover.

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Heat exposure during sleep could be measured as the number of hours when bedroom conditions exceed health-related temperature or humidity thresholds. Recovery could then be assessed using sleep quality, physiological measurements and next-day symptoms or performance.

Low-cost sensors can record bedroom temperature and humidity every few minutes^8,9. In field studies, sensors can be placed in bedrooms while participants wear wrist devices that track sleep timing, sleep continuity, movement, skin temperature, heart rate and heart-rate variability^8,9. Morning blood pressure, fatigue, sleepiness and alertness can also help determine whether overnight recovery was complete.

Skin sensors and electrodes can provide more detailed measurements of core body temperature and sleep stages. Smaller studies using these methods could help validate larger population-based investigations^5,8. With informed consent, researchers could also connect participant data with healthcare records to examine hospitalizations and other health outcomes.

These methods are already technically feasible and have been used to study how air pollution affects sleep¹⁰. Research shows that warm, poorly ventilated bedrooms are linked to poorer sleep quality, particularly among older adults^8,9. The HeatSuite platform has also demonstrated the feasibility of combining environmental sensor data with wearable measurements, including blood pressure and body temperature¹¹.

These approaches should be expanded to participant groups monitored before, during and after hot nights. Repeated measurements are essential for determining whether incomplete overnight recovery creates a physiological burden that lasts for several days. Studies should collect environmental, sleep and physiological data over multiple weeks, supported by brief daily surveys about symptoms, medication use and cooling behaviors.

Such research could clarify how incomplete overnight recovery contributes to higher mortality and hospitalization during hot nights^2,3. It could also reveal the health effects of repeated moderately hot nights and determine whether real-time indoor temperatures are more relevant to health than outdoor minimum temperatures.