It always goes the same way: after just a few weeks of serious sunshine, the world—outside of our air-conditioned bubbles—turns into an oven. You, who maybe just a month ago were confidently running at a steady pace, now find yourself puffing like an old steam engine, your legs refusing to move the way they should. You wonder: am I getting worse, or is it really just the heat?
The answer is simple: it’s physiological. And next time you start worrying that you’re suddenly falling apart, you’ll know that’s not the case.
The Internal Thermostat That Slows Us Down
When you run, your body produces heat. A lot of it. According to the American College of Sports Medicine (ACSM), up to 80% of the energy generated during a run is dissipated as heat. That’s an incredibly efficient system, considering that humans are built to sweat.
Sweat is our natural air conditioning: as it evaporates, it cools the skin and lowers core temperature. But when the air is already hot and humid, this process slows down. Evaporation becomes less effective, and your body accumulates heat.
The result? Your autonomic nervous system sends signals to ease off the effort and protect your body.
The Brain Does Its Job (Even if We Don’t Like It)
How does this all work? The Journal of Applied Physiology explains it well: fatigue isn’t just about your muscles— it’s largely a perception regulated by the brain. When your body temperature gets too high, the brain kicks in with a self-preservation mechanism: it slows down the signals to your muscles to prevent dangerous overheating.
In short, it’s not that you’ve lost your edge—your nervous system is literally saving your life by forcing you to slow down.
Learning to Read the Signals
How can you tell if you’re slowing down because of the heat and not because you’re out of shape? Here are some clear signs:
- Increased perceived effort: running at your usual pace suddenly feels much harder.
- Higher heart rate: even at an easy pace, your heart rate climbs.
- Heavy sweating (or sometimes reduced sweating, if you’re already quite dehydrated).
- Early onset of fatigue compared to what you’re used to.
In these situations, it’s smart—and totally normal—to slow down. Accepting this will save you from unnecessary frustration.
This Is Not the Time for a Personal Best
It’s easy to forget that your running performance in summer simply can’t match what it is in spring or fall. Environmental conditions matter—a lot.
For every degree above 20°C (68°F), your 10K time can slow by about 1–2%. On longer distances, the impact is even greater. And no, this isn’t about willpower—it’s pure physiology.
Even elite runners know this: during summer races, they rarely aim for records. They adapt their strategy instead, focusing on managing the effort rather than pushing hard.
Listen, Adapt, Accept
That’s the key word here: accept. Is the heat slowing you down? Good. That’s normal. Your body is speaking to you. You can choose to listen and still have a valuable training session, or you can stubbornly fight it—and risk ending up wiped out (or worse, sick).
Personally, every summer I find myself relearning this dialogue. My pace drops, my heart rate rises, sweat becomes my constant running companion. But there’s something beautiful in all of this: learning to understand yourself, to take care of yourself, to realize that progress isn’t only measured by the numbers on your watch.
So next time you head out for a run and the heat feels like an invisible wall, remember: it’s not you slowing down. It’s your body working *for* you, not against you.