Why hot nights can be worse for you than hot days

People dealing with record summer heat waves across the United States and Europe aren’t getting much of a respite when the sun goes down. Climate change is causing temperatures to stay high at night, and climate scientists and health experts say hotter evenings may lead to social and medical consequences.

Why are hotter nights a problem?

Intense heat at night is “raising the risk of heat exhaustion and heat stroke even higher, especially for people without access to air conditioning,” said NPR. In cases where the temperature after sundown doesn’t drop below 75 to 80 degrees Fahrenheit, people are “not getting the opportunity for recovery overnight, as we’ve historically seen,” Ashley Ward, the director of the Heat Policy Innovation Hub at Duke University’s Nicholas Institute, told NPR.

It is largely the fault of climate change that no nighttime recovery is occurring, as “each summer we are seeing more heat waves similar to the current one and also more hot nights,” said NPR. The “number of nights where the temperature never falls below 70 degrees Fahrenheit is increasing everywhere in the U.S. except the Northern Great Plains,” according to 2023’s Fifth National Climate Assessment. In fact, the extent of the continental U.S. “experiencing hot summer nights is growing at a faster rate than the extent experiencing hot summer days.”

Many people also find sleeping in hot weather difficult, which could further contribute to negative health effects. A June heat wave in the United Kingdom resulted in nearly 65% of people saying they “struggled to sleep,” according to a YouGov survey, while about half said they lost at least three hours of sleep per night. When this happens, the “implications are significant: a drop in work performance, an increase in accidents, lower school test scores” and a “decline in mental health,” Dr. Laurence Wainwright from the U.K.’s University of Oxford told The Guardian.

The type of residence in which people sleep can also make a difference. In Philadelphia, hotter nights are becoming “more dangerous than the days, particularly for older people who live alone in brick rowhouses in the city,” said The Philadelphia Inquirer. Without cooler temperatures, these rowhouses “can become ‘brick ovens’” at night and lead to heat-related deaths. Philadelphia’s dense neighborhoods can also be “several degrees warmer than other areas even within the city,” further exacerbating the problem.

What can be done about this?

With the continually changing climate, nights will likely remain hot, especially during summer. A 2022 Lancet Planetary Health study “projected that heat-related deaths could rise sixfold by the end of the century, largely because of warmer nights, unless planet-warming pollution is significantly curbed,” said The Sacramento Bee. Heat currently “kills more Americans in an average year than any other weather hazard.” The National Oceanic and Atmospheric Administration’s 10-year range from 2014 to 2024 recorded an average of about 250 heat-related deaths annually.

Experts say there are warning signs you can look out for to prevent overheating, whether during the day or night. “In its early stages, heat-related injury can be subtle, so it can be fatigue, increased thirst, nausea, vomiting sometimes and oftentimes muscle cramps, which is a symptom of dehydration,” the Cleveland Clinic’s Dr. Bryan Baskin said to AccuWeather. People “often think that only those out working in the environment, athletes, people who are outside all the time are at risk, and it’s really not true.”