Why hot nights can make a heat wave more dangerous

When indoor temperatures remain high, the body has to keep working to cool itself, even during sleep.

Heat waves can turn dangerous long after the sun goes down.

While afternoon high temperatures often get the most attention, AccuWeather meteorologists and health experts warn that hot and humid nights can be just as dangerous, especially when the heat lasts for several days and homes do not cool down overnight.

“What is often lost on heat waves, we focus on the day temperatures, but the problem is when you get to dew points and the humidity, it never cools off at night, and that is what makes heat waves deadly,” AccuWeather Meteorologist Bernie Rayno said.

Heat is the No. 1 weather-related killer in the United States, and prolonged heat can become especially hazardous when the body has little opportunity to recover.

On AccuWeather’s HeatWave Severity Index™, the danger is not measured only by peak afternoon temperatures. The index also highlights prolonged risk, including hot and humid conditions that persist overnight and can increase the threat of heat-related illness.

Emergency Room Dr. Bryan Baskin, with the Cleveland Clinic, told AccuWeather that people can develop heat-related injuries even while inside their homes.

“We see many people with heat-related injuries from homes that overheat, especially into the evening hours when they're staying immobile in a bed space, sleeping, sometimes with covers on themselves,” Baskin said.

Hot nights can be particularly dangerous for older adults, young children, people with chronic medical conditions and those without reliable access to air conditioning. When indoor temperatures remain high, the body has to keep working to cool itself, even during sleep.

“It's still a place to have caution, especially with the multiple days of heat over the next week, it really can build on itself, as can that dehydration put people at risk,” Baskin said.

The risk rises when humidity is high. Sweat is the body’s natural cooling system, but when the air is loaded with moisture, sweat evaporates more slowly. That makes it harder for the body to release heat.

“Body temperature typically 98 F, but when the temperature reaches above 90 degrees and you add humidity, it decreases our own body's ability to release its heat into the environment and cool itself,” Baskin said.

Once the body starts to overheat, the danger can escalate quickly.

“As the body temperature heats up, it can also lose its ability to regulate its own temperature, and that cycles on itself. And so that's when we see heat exhaustion and then heatstroke or heat illness and injury,” Baskin said.

Knowing the signs of heat illness before a trip to the ER

Early signs of heat-related illness may not be obvious at first.

“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,” Baskin said.

Confusion, agitation, fainting, worsening weakness or a very high body temperature can signal a medical emergency. Heatstroke can be life-threatening and requires immediate care.

Baskin said one dangerous misconception is that heat illness only affects people who spend long periods outside, such as athletes, construction workers or landscapers.

"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," he told AccuWeather. "Many times those people are acclimated to the hot temperatures, the humidity, and, and do OK as long as they don't overexert themselves."

People indoors can also face serious risks when buildings trap heat over multiple days.

"Those that are inside often, they're at risk, and even sometimes people that are in their homes that heat up over the course of the week, even without going outside, they're at risk too," Baskin said.

During prolonged heat, people should try to spend time in air-conditioned spaces, drink water throughout the day, check on vulnerable family members and neighbors, and avoid relying on fans alone when indoor temperatures are dangerously high. A cool shower, lightweight clothing and limiting oven or stove use can also help keep indoor temperatures lower.

The key is not to underestimate the danger after sunset. When nights stay hot and humid, the body may never get a break, allowing dehydration and heat stress to build day after day.

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