How Your Body Adapts to Changing Temperatures

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The first heat wave of summer hits like a wall . One day the air is crisp and cool, and the next it’s impossible to go out without sweating through a shirt. The store feels like it’s on the far side of the Amazon, or maybe the Gobi. And yet, the thermometer reveals a number that is not all that high, compared to later in the season. So why is the heat so unbearable ?

Scientists who study temperature regulation have found that the body takes time to adjust to seasonal changes and other thermal changes: a process called heat acclimatization. Research suggests that in people who regularly spend time outside, the body adjusts how it sheds heat to make the environment less stressful . “We can adapt to the heat we're in, and it's obviously progressive,” says Julien Périard, a professor at University of Canberra in Australia who studies the phenomenon. Your body in the summer, it turns out, might be fundamentally different from your body in the winter.

How does the body adapt to seasonal temperature changes?

Spending most of your time indoors in the air conditioning might slow the body’s changes, come summer. “But if we spend time outside exercising, or even just being physically active outdoors, we'll adapt to those conditions,” Périard says.

When you go for a walk on a hot summer day, before you’ve adapted to the heat, both your skin temperature and your core temperature may go up . That sets off alarms in the body, announcing heat stress. “With that, we trigger lots of sweating, and we increase our skin blood flow,” says Périard. Sweat evaporates from the skin, cooling skin down, and blood sent to the surface of the body helps shed heat.

The volume of blood pumping through the body also goes up. More blood volume allows more heat to be shed and supports greater sweating without dehydration. There are also changes at the level of the cell, with some proteins’ production going up to protect normal functioning in greater heat. The process of reaching a fully adapted state might take a few weeks, although the precise details will depend on the situation.

These changes can be triggered in advance, with a little bit of exposure at a time. In fact, athletes and people working in warm conditions, like utility workers out in the Texas sun all day, may slowly ramp up their time in the heat to stay healthy under strain.

Does adapting to heat reduce the risk of heat stroke?

Being adapted to the heat lowers the risks . But it isn’t carte blanche to go for a run at noon in Havana. People who’ve adapted are still vulnerable to heat-related illness, like heat stroke and heat exhaustion, says Glen Kenny, a professor of physiology at the University of Ottawa in Canada.

What’s more, researchers who study heat adaptation are worried about the stresses that the future climate will bring, when warming may be so swift, and so extreme, that the body can’t react fast enough. In an experiment meant to mimic an extreme 3-day heat wave in 2021 that killed 619 people in British Columbia alone , Kenny spent three days at 104°F in a special chamber in his lab, which allowed him and his colleagues to monitor what happens to the body. “Over three days, you'll see that the body will show some progressive adaptation. But in this situation, you're seeing a decay in the physiological system supporting the maintenance of core temperature,” he says. He lost about 10 pounds in three days because he couldn’t hydrate fast enough. “You're losing so much sweat. You're seeing cell function deteriorate.”

What happens if you lose your heat adaptation?

Spending a few weeks in a cooler climate can mean the body will roll back some of these changes. “But what's interesting is we have some sort of heat acclimation memory ,” says Périard. “If within a month, for example, I've removed you from the heat, and then I've put you back in the heat, in three or four days you'll probably get adaptations that are similar” to what would have taken ten days to build up originally.

Does your body adapt to the cold, too? Yes, but there’s a lot less to work with, says Périard. “We've evolved to be tropical animals, so our heat-loss mechanisms are very potent. But if you think of conserving heat, the only two things we can do are physical constriction—to try and keep the warm blood in the central circulation, essentially—or shivering, which produces a bit of heat but not too much.” Exercising can warm you up, but an essential feature of human culture—clothing—has a far more important role to play here than biology.

“You can exercise all you want. But if it's -40° outside, you're going to get cold pretty quickly if you don't have any clothes on,” he says.