Hotter Nights, Brought on by Climate Change, Pose More Health Threats
When nights stay hot, more people die, many from cardiovascular problems. But there are simple methods you can use to stay cooler and healthier
This article was made possible by the support of Yakult and produced independently by Scientific American‘s board of editors.
If the summer of 2025 is anything like last year’s, get ready to sweat. July and August of 2024 were among the hottest months on record in the U.S. Phoenix, Ariz., saw daytime temperatures higher than 100 degrees Fahrenheit for more than 100 days. But now temperatures are staying high at night, too, which increases health dangers. When it stays hot after the sun goes down, more people die than typically would. Emergency room visits and hospital admissions go up. Premature births increase. Sleep and mental health suffer. But there are simple and practical methods you can use to stay cooler and healthier, beyond cranking up your expensive air conditioner.
“Most heat exposure is chronic,” says Ashley Ward, director of the Heat Policy Innovation Hub at Duke University’s Nicholas Institute. She and others are increasingly focused on the ***ulative effect of warmer nights following warmer days. Nighttime temperatures are rising twice as fast as daytime temperatures because increasing cloud cover at night, created by the greenhouse effect, traps heat and sends it back to the ground. In parts of the Southeast, for example, there are now more than 30 days a year when the temperature stays above 75 degrees F at night, Ward says. Urban heat islands, which are parts of cities with lots of concrete and few shade trees, trap warmth and add to the effect, but rural areas are also suffering.
On supporting science journalism
If you’re enjoying this article, consider supporting our award-winning journalism by subscribing. By purchasing a subscription you are helping to ensure the future of impactful stories about the discoveries and ideas shaping our world today.
“If it doesn’t cool down at night, then your core body temperature can’t really get back to what is normal for you,” says epidemiologist Kristie Ebi of the Center for Health and the Global Environment at the University of Washington. “You’re starting the next morning with a higher baseline.” That’s why death rates start to increase after about 24 hours during heat waves. “It’s not the instantaneous exposure; it’s the buildup over the course of a day, not getting relief at night. That starts affecting the cells and organs,” Ebi says.
Core body temperature is important because it’s the heat of your internal organs rather than your more obvious skin temperature. “Heat is called a silent killer because humans are not terribly good at determining if their core body temperature is rising,” Ebi says. “It has to stay within a narrow range to protect us and our organs.” That range is roughly from 97 to 99 degrees F, but it varies throughout the day and from person to person.
When your face turns red in the heat, it’s because your body is pushing blood to the surface to cool it. Likewise, when sweat evaporates from the skin, it takes body heat with it.
Bodies are designed to cool down through tricks such as flushing and sweating. When your face turns red in the heat, it’s because your body is pushing blood to the surface to cool it. Likewise, when sweat evaporates from the skin, it takes body heat with it. But the body can’t always keep up. The process of thermoregulation can strain the heart, which must work harder at pumping blood, and harm the kidneys, which can be injured by dehydration.
Sleep habits, critical for overall health, suffer in the heat, too. Recent global studies using survey data and billions of measurements from fitness bands found that people sleep less well as temperatures go up, says Nick Obradovich, chief scientist for environmental mental health at the Laureate Institute for Brain Research in Tulsa, Okla. Heat makes it harder to fall asleep and causes people to wake a little earlier than usual. “It increases the probability that people have what we define as short sleep, or less than seven hours a night,” Obradovich says. Furthermore, the more hotter nights pile up, the worse people sleep. Inadequate sleep damages the immune system and is linked to higher risk for cancer and Alzheimer’s disease. It is also tied to poorer mental health.
The emotional and behavioral problems blockociated with rising temperatures include an increased risk of suicide. Again, the longer those temperatures persist, the stronger the psychological consequences. In general, “you’re much more likely to have a mental health episode by the sixth day [of a heat wave] than you are on the first day,” Obradovich says.
To make matters worse, rising temperatures are colliding with poverty, age, and other social determinants of health, Ward says. Many people in the least energy-efficient housing are also least able to afford air-conditioning. Older adults are more likely to have underlying heart or artery diseases, diabetes or respiratory illnesses that make them more susceptible to heat. Prescription drugs, such as those used to treat Parkinson’s disease and beta blockers used for hypertension, reduce the body’s ability to thermoregulate.
Geography matters, too. High humidity, often found in the southeastern U.S., is especially dangerous because it makes it harder for sweat to evaporate. In the dry heat of the Southwest, sweat evaporates fast, dehydrating you before you realize it.
Fortunately, people can help their body make the most of its natural cooling strategies with some scientifically proven tactics. Immersing your hands and feet in cool water—not icy cold—triggers a response from special types of blood vessels that release internal heat to the surface, where it can dissipate fast. The method is used regularly by the military. And after a hot day of yard work, Ward says, it’s much more effective to take a cool shower than to open a cold beer—the former will lower body temperature, whereas the latter is a diuretic. Staying hydrated (with water!) reduces strain on the kidneys. If access to air-conditioning in your home is limited, concentrate on lowering the bedroom temperature rather than other rooms. That will offer you a better chance to sleep, perchance to dream of cooler nights to come.
