Experts are sounding the alarm over the effects of overheating on the planet, as periods of heat stress from heat waves grow longer and longer, taking a toll on sleep and health overall. Temperatures now tend to stay high even after sunset, and nights no longer provide the necessary breathing room of cool air the human body needs to recover.
The planet is heating up, a Stanford University study warns, at a time when Europe and America are experiencing an unusually hot summer. In France alone, unprecedented heat waves have already been recorded since late June, with excess mortality surpassing 6,000 deaths. Data from the Copernicus climate change service shows that the minimum temperature on the year’s hottest nights has been steadily rising for decades, as has the frequency of hot days followed by hot nights.
The effects on the human body are far from negligible. When the normal nighttime drop in core body temperature is disrupted, the heart is put under strain, sleep is disturbed, and the body starts the next day already exhausted. This means that as temperatures rise, so does the risk of poor sleep.
Unlike daytime heat waves, where authorities mainly focus on safety once temperatures pass 35°C (95°F), the effects of hot nights on sleep and health aren’t yet fully understood.
A recent study using Copernicus data found, as reported by Euronews, that nights are warming especially fast. Since 1970, the minimum “feels-like” temperature during the ten hottest nights of the year has risen by 0.32°C per decade globally.
This figure is based on the Universal Thermal Climate Index (UTCI), which factors in temperature, humidity, wind, and radiation. The number of tropical nights, when temperatures don’t drop below 20°C (68°F), has also increased by roughly two nights per decade.
The Problem With Hot Nights
Researchers say this upward trend shouldn’t be overlooked, since hot nights give the body fewer chances to recover from the heat it endured during the day. This becomes even more of a concern as hot days followed by hot nights grow more frequent, a pattern known as “compound heat wave events,” which have increased by 73% in Europe since the 1970s.
The problem can become especially 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, right as temperatures begin climbing again. Researchers are increasingly focused on whether this repeated lack of nighttime “recovery” could help explain why prolonged heat waves pose such a serious health risk.
A hot night can turn out to be just as dangerous as a hot day, perhaps even more so. During sleep, heart rate and physiological activity are supposed to slow down while core body temperature drops, allowing the nervous, immune, and cardiovascular systems to recover from the day’s strain.
But if the room stays too hot, the body struggles to release that heat. Blood vessels near the skin dilate to release warmth, forcing the heart to work harder to circulate blood, while sweating can lead to dehydration. The result can be more difficulty falling asleep, more frequent waking during the night, and less restorative sleep overall.
What You Can Do
The World Health Organization recommends keeping indoor temperatures below 24°C (75°F) at night where possible, to help the body cool down and support better sleep during heat waves.
However, most homes in regions unaccustomed to heat aren’t built for rising temperatures and may struggle to cool down after sunset. As a result, bedrooms can stay warm and humid long after the worst heat of the day has passed.
Northern Europeans are a notable example, still largely resistant to air conditioning. For decades, the priority in these countries was staying warm through especially cold winters, not protecting against extreme summer heat, so most buildings weren’t built with cooling in mind.
According to the data, Germany still has one of the lowest rates of air conditioning use in Europe, with only 6% of German households owning a unit. Still, that’s changing fast, along with the growing need for cooling. It’s no coincidence that demand for air conditioning in Germany rose by roughly 75% between 2019 and 2024.
In any case, to sleep better on hot nights during a heat wave, here are seven ways to help cool down your home:
- Open windows after dusk if the air outside is cooler than indoors.
- Close blinds, shutters, or curtains during the day to keep the room from heating up.
- Use lightweight bedding and loose, light clothing.
- Take a cool shower or bath before going to bed.
- Drink water regularly throughout the day, but limit alcohol and caffeine.
- Move to the coolest room in the house if your bedroom gets too hot.
- Turn off unnecessary electrical devices, which generate extra heat.
Older Adults Are the Most Vulnerable
While many earlier studies on heat wave effects focused mostly on younger, healthy people, it turns out older adults face greater risk, and their share of the population keeps growing.
The Stanford University study, published in Lancet Planetary Health, warns that older adults are far more vulnerable to heat than researchers previously believed. With the global population over 60 continuing to grow, the study’s authors estimate that extreme heat will affect far more people, over a wider area, and for longer stretches than previously calculated.
The result is a vicious cycle: the group most at risk from heat is also the group with the least nighttime “breathing room” to recover.
The issue centers on a key metric, temperature combined with humidity, technically known as wet-bulb temperature (Tw), a combination that can become especially dangerous for the human body. Cross-referencing new data with projections for global population aging over coming decades, the American researchers concluded that unbearable heat will affect far more people, over wider areas, at a much larger scale, and for longer durations than previously estimated.
The researchers stress that the countries at greatest risk are those where large portions of the population have no way to protect themselves from heat, underscoring the need for measures that improve access to tools like air conditioning, alongside cutting the carbon emissions driving the temperature rise.
