Nighttime warming threatens the sleep of billions

As the world heats up, nights are warming faster than days where most people live—and this ambient heat affects how well and how long people sleep. A new article by eminent sleep scientists, including the presidents of the World Sleep Society and International Pediatric Sleep Association, suggests that warm nights are already degrading sleep for billions of people worldwide. The problem is poised to become significantly worse if this trend continues without further adaptation.

“We now see converging global evidence that even moderate increases in nighttime heat push even well-off populations to sweat through clinically short nights of sleep, which carry real public health consequences,” says Kelton Minor, Associate Professor at the Department of Psychology and Copenhagen Center for Social Data Science, University of Copenhagen and lead author of the article published in the journal SLEEP.

Today’s best estimates suggest that for every 100,000 people exposed to a hot night (∼27°C or 81°F), about 9,300 additional people sleep less than the six hours they would on a normal night.

Impact least known where heat and poverty are highest

The study reviews more than a decade of research into the link between temperature and sleep. The researchers document robust evidence from both controlled laboratory experiments and large-scale natural experiments across multiple countries that warm nights actually cause people to fall asleep later and sleep for shorter periods. Worryingly, the authors find that previous research does not at all represent humanity, let alone those parts of the world where heat-related problems are greatest.

“It is paradoxical that we know the least about sleep among people in some of the world’s already hottest and most climate–vulnerable regions—precisely where ongoing temperature rises are expected to hit hardest,” says Minor.

He points to recent evidence from a survey of houseless individuals in Delhi, India, that found about half (49%) slept less than four hours a night, with nearly all (95%) blaming heat. One respondent stated, “The pavement gets so hot from the sun all day that it remains heated through the night. When we lie down, the heat seeps into our bodies, making it impossible to sleep.”

People from Africa and South-East Asia, in particular, are massively under-represented in existing studies. These are areas that contain an outsized portion of the current and future human population, and that are already experiencing heat levels and temperature increases that exceed the global average.

The built environment can retain heat

The researchers show that different adaptations, including access to air conditioning and gradual acclimatization to heat over time, do not appear to adequately or reliably shield sleep from the harms of heat.

In fact, the environment humans have constructed may be making matters worse. Converging lines of evidence suggest that a local temperature rise does not just affect sleep loss on the first night, but can continue to do so for several days. This means that just a few hot nights can cause sleep deprivation to accumulate for much longer.

“The evidence suggests that heat does not just make it harder to fall asleep on the night of. The built environment appears to trap sleep-harming heat inside even after elevated temperatures subside outside. Such prolonged insufficient sleep poses a well-known risk for physical and mental health harms shown in independent research to also increase on hot days,” says Minor.

Poorer sleep can also impair people’s neurocognitive function and general performance, and the phenomenon also increases the risk of accidents and vulnerability to illness and infection.

“Put simply, human sleep appears far from fully adapted to the warming climate we already experience today, let alone the much hotter one our children will inherit,” says Minor.

A ‘wake-up call’ for a global task force

The article points out that sleep research has long focused more attention on other environmental variables such as lighting while ignoring or fixing temperature constants. However, recent observational evidence made possible by the advent of wearable and nearable devices shows that, in the real-world, ambient heat can harm sleep substantially over very large areas, and often for prolonged periods of time.

The authors point out that if current trends continue without adequate action, sleep impacts due to hotter temperatures are expected to not just increase, but accelerate.

“The human sleep response to temperature appears non-linear, such that the hotter the outdoor temperature is the more sleep that is lost for every additional degree of warming,” says Minor.

Based on the growing evidence, the researchers are calling for the establishment of a global climate and sleep task force to improve monitoring and promote adaptation where it is lacking, while unifying knowledge across countries and research communities.

“We need a coordinated global effort to understand how Earth’s climate system affects sleep across life stages and population groups—and to develop solutions that can rapidly scale to protect our nights in a warmer world,” says Minor.

He emphasizes that maintaining sleep health in a warming climate is not merely an individual concern, but a key public health factor that should be incorporated into everything from sleep hygiene, housing policy, architectural design, urban planning, sleep medicine, to global climate adaptation and action plans.