Nobody likes to feel sluggish and sweaty, so when the sun is set to “broil,” a lot of us would rather take our workout to an air-conditioned gym. But the human body can acclimate to exercising in the heat, so it may be worth heading outdoors anyway. After a few weeks, these temperatures will be your new normal—and research suggests you may enjoy a small performance boost when the weather cools down again.

Hot workouts can be dangerous, so I trust that you know common-sense advice about running in the heat. Among the most important: Drink to thirst (or a little bit more), and stop and get help if you start feeling symptoms of heat illness like nausea, dizziness, or weakness. And while it’s great to work on your ability to run in the heat, don’t be stupid about it—stay inside if the temperature is hotter than you can handle, and stay aware of air quality levels (which get worse on hot days).

Why exercising in the heat feels so miserable

Running is miserable and heat is miserable; therefore, running in the heat is miserable. But there’s more to it than that, and exercising in the heat feels even worse than you'd expect from stacking those two factors together.

Exercise raises your body temperature, and when you stack that rise in temperature on top of the heat from the weather, it's very possible to reach dangerous levels of body heat. That means your body has to work harder than usual to cool itself down. The ways we cool ourselves also interfere with exercise performance. For example, your heart is working to supply blood and oxygen to your muscles, and to pump blood to the surface of our skin for cooling. Those tasks are a lot of work—no wonder we feel exhausted in the heat.

As a safeguard, your brain perceives effort differently in the heat, so even before you overheat, you feel sluggish. In a study published in the European Journal of Physiology, cyclists who worked out in a lab with a temperature of 95 degrees were slower than when they did the same time trial at 59 degrees. That makes sense, but here’s the weird part: They didn’t overheat and then slow down. They were slower from the start. It seems our brains slow our bodies down proactively on hot days in order to conserve energy.

As a workout continues, our bodies heat up. In another study, published in the Journal of Applied Physiology, when asked to cycle to exhaustion, participants pooped out when their core temperatures reached 104 degrees, no matter what temperature they started at. That's the temperature where serious heat illness can set in, so it makes sense that our bodies will put on the brakes at that point. In that study, athletes who kept cool with a fancy water-cooling jacket lasted the longest. (You can mimic this effect in your own workouts by drinking ice-cold beverages and pouring water over your head.) The longer you can keep your body cool, the longer you can keep up a hard effort.

It's the heat and the humidity

But cooling your body isn’t a complete solution. Dumping a cup of ice water over your head or putting one into your belly only provides momentary relief, and water-cooling jackets aren’t practical outside of physiology labs. So let’s look at what happens in real-world conditions.

Our most efficient way of cooling off is sweating. When moisture evaporates from your skin, it takes some body heat with it. In humid weather, though, sweat doesn’t evaporate as easily because the air is already full of water vapor. So when we’re talking about “heat,” we really mean something more like “perceived heat,” which is a combination of heat and humidity. This heat index chart shows the relationship:

You’ll run slower in the heat (and humidity). While you can find charts like this one that predict how much slower you will run a race, the truth is that heat's effect on your running depends on whether you’re used to the heat, and on your body size.

That’s right—not your fitness level, but your actual physical size. People who are larger have more muscle, fat, or both. Muscle generates heat, and fat acts as an insulator. On the other hand, smaller folks generate less heat, but have more skin through which to dissipate that heat—the ol’ surface area to volume ratio. This is why petite runners place better in races on hot days.

Some people think being more fit makes you better at dealing with heat, but that's only partially true: The fitter you are, the more body heat you produce, just because you’re so good at working hard. Short of changing your body shape (which is possible, but hardly a short-term fix), what can you do to tolerate exercising in the heat better? The answer is simple: Spend more time exercising in the heat.

Why you should start a heat adaptation protocol

Running in the heat makes you better at running in the heat—and it makes you better, period. Say you do all your workouts outdoors this summer, while your equally fit twin does identical workouts on a treadmill in an air-conditioned gym. Who do you think will finish first in a 5K on a hot weekend in August? That’s right, you will.

But even if the weather is unseasonably cool on that August day, your heat training will still help you beat your twin. Part of the magic of heat training is that it increases the amount of blood in your veins (the better to put it towards your skin for cooling, while still having enough to fuel your muscles). The effect has been compared to a mild, totally legal version of blood doping. Scientists are still debating exactly how this effect works and whether it always happens when people attempt to adapt to heat, but overall, the evidence is strong enough that I believe we should all try to get some of those adaptations if we can do so safely.

Here’s the bad news: Heat adaptation takes work. It’s not enough to sit around in the air conditioning all summer, only venturing outside for occasional workouts. A study published in the European Journal of Applied Physiology found that people who made no particular effort to exercise in the heat didn’t have any better heat tolerance in the fall than in the spring. If you want the advantages of heat training, you have to work for them.

I'll go over some heat adaptation strategies below, but the general idea is that you spend at least two weeks making sure to get plenty of time in the heat. Your body will get better at cooling itself, and those hot-weather runs will feel a little less awful. Heat adaptation isn't magic, though. You'll still feel hot, and you may still run slower than in cool temperatures. You'll just be able to get more of a workout in before your body temperature rises to dangerous levels. If you want to know more about heat training, I wrote about my experience last year with a heat-training sensor, which allowed me to get detailed feedback about how my heat training was going.

How to adapt to exercising in the heat

Option 1: Train normally, but without avoiding the heat, for two weeks

In scientific research, heat acclimation protocols for athletes generally involve 7 to 10 consecutive days of exercising in the heat, for 60 to 90 minutes each day. A simpler method that can be done by entire teams, or by individuals, is to simply do your normal training in the heat for about two weeks.

Take it easy at first. Remember that your body is still trying to convince you that you are super tired and need to slow down. Safety guidelines for workers provide a reality check here. On their first day in the heat, the Occupational Safety and Health Administration (OSHA) recommends giving workers just 20% of their usual workload. Within a week, they should ramp up to 100%.

Option 2: Spend two hours in the heat every day, whether you're exercising or not

This U.S. Army training protocol provides a good road map for adapting yourself to the heat: Spend at least two hours in the heat each day, it says, and include cardiovascular exercise (like running, cycling, or anything that gets your heart rate up) as part of that. If you can’t handle two hours without feeling symptoms like nausea or dizziness, do what you can and view the two-hour benchmark as a goal to work toward.

You can expect to be better adapted to the heat after about two weeks of spending two hours per day in the heat, although you may be able to start seeing results in just a few days.

Option 3: Visit the sauna after each workout

A way to combine the protocols above, without relying on the weather, is to step into a sauna or hot bath after your workout. If your gym has a sauna, this is a great way to use it. The post-workout sauna time will help your heat adaptation, and you can do it even if your workout was done in less-than-sweltering temperatures.

The time spent in the sauna can be 15 to 30 minutes, starting with a shorter time and working up to longer. Note that these targets only apply if you're using the sauna after a workout when your body temperature is already elevated from exercise. If you skip the workout, you'll need to add 20 minutes or so to your sauna time to get the same effect.

How to keep your heat adaptations even when the weather cools off

To stay adapted to the heat, you have to keep spending time in it. You can take a few days off, but if you slack off for a week, you’ll start to lose your hot-weather superpowers. This snowballs quickly: you'll lose about 75% of your adaptations after three weeks, according to the Army’s estimates.

To keep up your heat training in cool weather, you can try wearing long sleeves and tights, like elite runner Kara Goucher did when training for a world championship race in muggy Osaka. (She won a bronze medal, the first American ever to do so). She also spent a few weeks in Osaka before the race began; traveling to experience the heat might be an option worth considering if you’re a dedicated athlete with vacation time to burn.

You can also try the opposite of all the standard keep-cool advice, and choose to run at the hottest time of day on asphalt roads with no shade. Or return to the sauna protocol after your cooler-weather workouts. Whatever you do, stay safe, and enjoy your new superpowers.