The Science Behind Why Heat Waves Make Us So Tired

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Blazing sun and soaring temperatures affect your skin, heart, and muscles in ways that sap your energy

By Alex Janin- WSJ

When hot days leave you feeling tired this summer, the sensation isn’t all in your head: There’s real biology behind your lethargy.

Soaring temperatures this month set records for the earth’s hottest days and sapped people’s energy levels. Exposure to hot surroundings can jump-start a series of physiological processes in the heart, brain, skin, and muscles that can leave you mentally and physically drained.

“Fatigue is the body’s way of telling us to rest,” says Dr. Meagan Wasfy, a sports cardiologist at Mass General Brigham in Boston. “A way to mitigate the impact of heat is to move your body less.” 

The human body is generally less tolerant of heat than cold, she adds, though people can acclimate somewhat to extreme temperatures on either side of the spectrum. Your body’s mechanism for increasing core temperature when you’re cold, such as by shivering, works more efficiently, she says. Very high body temperatures pose a risk of major damage or failure to organs, whereas functionality may slow down but bounce back more easily when the core body temperature is low, she says.

Here’s what doctors and scientists say is going on when your energy flags under a hot sun.

What happens outside your body

You can feel the effects of radiant heat, which comes from electromagnetic waves like the sun’s, even if the air temperature feels physically cooler. PHOTO: ISTOCK

The number on the thermometer is important, but it doesn’t solely determine how your body will respond to heat. Humidity matters, too. Sweat evaporates off your skin into the air to cool you off. That becomes more difficult in humid conditions when the air feels wetter.

Radiant heat, which comes from electromagnetic waves like the sun’s, also makes a difference. You can feel the effects of radiant heat even if the air temperature feels physically cooler.

What happens inside your body


When your skin heats up, blood vessels near the surface widen to allow more blood to flow up to the skin for that heat to dissipate. “You’re turning those blood vessels up from like a one-lane country road to a four-lane highway,” says Wasfy. This process, called vasodilation, reduces blood pressure, which can make you feel tired. You also produce more sweat as temperatures rise, which allows the skin to cool as it evaporates.


Extreme heat mimics the effect of exercise, where your heart has to work harder than normal, says Dr. John Schumann, a physician and executive medical director of primary care network Oak Street Health. Vasodilation and sweat production take energy. Your heart rate typically increases in the heat to compensate for the body’s increased metabolic demands.


More blood flowing directly to your skin leads to less flow to the muscles, which can contribute to fatigue or feeling weak. Exercising when it’s hot can produce a double-whammy effect, where more blood flow is going to your skin and muscles and less is available for vital organs.


The heat can make you feel not just physically tired, but mentally fatigued, too. Studies have linked heat waves to slower reaction times, delayed working memory, and reduced productivity among students and adults. In one study published in 2018, students in buildings without air conditioning in a heatwave experienced 13.4% longer reaction times on one type of cognitive test than those with air conditioning.

Scientists don’t fully understand why, but “we know that when the brain temperature is elevated, the amount of work or effort conscious to do a job is elevated,” says Dr. Craig Crandall, a thermal physiologist based in Dallas who studies the effects of heat on the body.

Sleep and hydration

How much water you drink affects energy levels.

Another driver of fatigue during extra-hot days: restless nights. People tend to sleep slightly less with each degree of temperature increase, a 2022 study showed. When nighttime temperatures exceeded 10 degrees Celsius, or 50 degrees Fahrenheit, outside, people slept 37 seconds less per night on average for each 1-degree Celsius increase, says the study’s lead author, Kelton Minor.

“You and I and everybody sleep better when it’s a little bit cooler than a little bit warmer,” says Crandall.

How much water you drink affects energy levels, too. You lose more water through sweat production when you’re hot, so you need to.