Running in Extreme Heat vs. Extreme Cold – What Happens to Your Body?

Chapter 1: How Your Body Handles Extreme Temperatures

Ever wonder why running on a blazing hot day feels like you're melting, but a freezing morning can make your muscles feel stiff and slow? That’s because your body is constantly working to keep its core temperature stable, no matter what’s happening outside. Whether you’re sweating buckets in the heat or shivering in the cold, your body has built-in systems to handle extreme conditions—though they work in completely different ways.

At rest, your core temperature hovers around 36-37°C (96.8-98.6°F). But when you start running, your muscles generate heat—a lot of it. In normal conditions, your body releases that heat efficiently. But when it’s too hot or too cold, things get more complicated.

Heat: Your Body’s Cooling System

In hot weather, your body fights to get rid of heat before it builds up to dangerous levels. It does this in two main ways:

Sweating – When you sweat, the moisture on your skin evaporates, cooling you down. But this only works if the sweat actually evaporates—high humidity can mess with this process, making you feel even hotter.
Blood Flow to the Skin – Your body moves more blood to the surface (your skin) to release heat into the air. That’s why your face gets red and your veins seem to pop out when you run in the heat.

The problem? These cooling methods come at a cost. Sweating leads to fluid and electrolyte loss, and redirecting blood to your skin means less blood going to your muscles, which can slow you down and make running feel harder.

Cold: Your Body’s Heating System

In the cold, your body has the opposite problem—it needs to keep heat in so your core temperature doesn’t drop too low. It does this by:

Reducing Blood Flow to the Skin – Instead of sending blood to the surface like in the heat, your body pulls blood away from your hands, feet, and face to protect your vital organs. That’s why your fingers and toes feel frozen first.
Shivering – When you shiver, your muscles contract rapidly to generate heat. It’s a last-resort method, because it burns a lot of energy and isn’t sustainable for long.

One tricky thing about cold weather running is that your body still produces heat when you move—sometimes too much. If you overdress, you’ll start sweating under your layers, which can leave you wet and chilled. That’s when hypothermia (dangerously low body temperature) becomes a risk.

Same Goal, Different Methods

In both extreme heat and cold, your body is doing everything it can to keep your core temperature stable. The challenge is that running makes things harder—it generates extra heat, forces your heart to work harder, and changes how your body uses energy.

So, what happens when your body can’t keep up? That’s where things get interesting—and risky. In the next chapters, we’ll break down what happens when you push your limits in extreme heat and cold, and how to make sure your body doesn’t lose the battle.

Chapter 2: Running in the Heat – Staying Cool is Hard Work

Running in the heat feels tough for a reason—your body is working overtime to keep from overheating. The problem? Staying cool takes a lot of energy, and if your body can’t keep up, things can go south fast.

Your Body’s Cooling System: Built for Survival, Not Speed

When you run in the heat, your body has one goal: don’t let your core temperature get too high. If it does, your organs and brain are at risk. To prevent that, your body has two main ways to dump heat:

Sweating – Your built-in AC system. Sweat evaporates off your skin, cooling you down. But this only works if the air is dry enough—high humidity makes it harder for sweat to evaporate, leaving you drenched but still overheating.
Moving Blood to the Skin – Your heart works harder to pump blood to your skin so heat can escape. That’s why you get red in the face and why your veins seem to bulge when running in the heat.

The Trade-offs: What You Lose to Stay Cool

Here’s where things get tricky—your body can’t cool down for free. Keeping your temperature in check comes with some serious trade-offs:

Less Blood for Your Muscles – Sending blood to your skin means less oxygen for your legs, making running feel harder. That’s why your pace slows down even if you feel fine.
Dehydration Happens Fast – You can lose a liter or more of sweat per hour, and with it, critical electrolytes (like sodium and potassium). Lose too much, and your muscles start cramping, your energy tanks, and your brain gets foggy.
Your Heart Works Overtime – To move blood to your skin while keeping your muscles going, your heart has to beat faster than normal. This adds to the exhaustion and makes every step feel like a struggle.

When Cooling Fails: Heat Exhaustion and Heat Stroke

If your body can’t keep up, your core temperature keeps rising—and that’s where danger kicks in.

Heat Exhaustion (Core Temp: 38-40°C / 100-104°F) – This is the warning stage. You feel weak, dizzy, nauseous, and your sweating gets out of control. Your heart is pounding, and even though you’re pushing, your legs feel like lead. If you don’t cool down, it gets worse.
Heat Stroke (Core Temp: 40°C+ / 104°F+) – This is when your cooling system completely fails. Your body stops sweating, your skin gets hot and dry, and confusion sets in. If untreated, this can lead to organ failure, brain damage, or worse. Heat stroke is a medical emergency—this is not the kind of toughness you want to test.

How to Run Smarter in the Heat

Since running itself creates heat, the trick is to manage how much you build up vs. how much you lose. Some key strategies:

Hydrate Early and Often – Start your run well-hydrated and keep sipping water. In long efforts, replace lost electrolytes too.
Wear Light, Breathable Clothing – The best gear wicks sweat and lets heat escape. Avoid cotton—it traps sweat and overheats you.
Slow Down – Your body is already working harder. Expect to run at a slower pace than in cooler temps, and that’s completely normal.
Run When It’s Cooler – Early mornings or evenings are your best bet. Midday sun? Not worth the struggle.
Know When to Stop – If you feel dizzy, nauseous, or lightheaded, listen to your body. No workout is worth heat stroke.

Heat is Tough—But Manageable

Running in the heat is hard because your body is doing two jobs at once: moving and cooling. The key is to work with your body, not against it. Train smart, hydrate well, and understand the warning signs—because in extreme heat, your body has limits, and pushing past them isn’t about mental toughness. It’s about knowing when to cool down.

Chapter 3: Running in the Cold – Keeping Warm Without Overheating

Running in the cold sounds simple—just layer up and get moving, right? Not exactly. Your body has to work twice as hard to keep your core warm while making sure you don’t overheat once you start pushing the pace. It’s a constant balancing act between holding onto heat and not sweating too much—because in cold weather, sweat can turn into a serious problem.

How Your Body Reacts to the Cold

The moment you step outside into freezing air, your body goes into heat conservation mode. The goal? Keep your core warm—even if that means sacrificing warmth in your hands, feet, and face. Here’s how it does that:

Less Blood to Your Limbs – Your body pulls blood away from your fingers, toes, and skin to protect your vital organs. That’s why your hands and feet go numb so fast.
Shivering – If you’re standing still, your muscles contract rapidly to generate heat. It works, but burns a lot of energy—making you tired even before you start running.
Tighter Blood Vessels (Vasoconstriction) – Your veins shrink to hold in heat, which helps keep your core temperature up. But it also makes your muscles feel stiff, which can make the first few minutes of a run feel sluggish.

Running Generates Heat—Sometimes Too Much

Once you start moving, your muscles produce a ton of heat. That’s great because it keeps you warm, but it also means you might overheat if you dress too warmly. Unlike in hot weather, where your body cools down by sweating and sending heat to the skin, in cold weather:

Sweat can make you colder – If you overdress and start sweating, that sweat sits against your skin. Once you slow down, it cools fast and can drop your body temperature too much, putting you at risk for hypothermia.
Your cooling system is slower – Because your body is trying to keep heat in, it doesn’t send as much blood to the skin. That makes it harder to release excess heat, meaning you can overheat faster than you expect.

The Risk of Hypothermia and Frostbite

If your body loses too much heat, you could run into real trouble:

Hypothermia (Core Temp Below 35°C / 95°F) – This happens when your body can’t produce heat fast enough to make up for what it’s losing. You start feeling sluggish, confused, and uncoordinated—a dangerous combo for a trail run far from help.
Frostbite – If your skin gets too cold, ice crystals can form inside your cells, damaging tissue. Fingers, toes, nose, and ears are at the highest risk. If they go numb for too long, that’s a bad sign.

How to Run Smarter in the Cold

Since your body is juggling staying warm and not overheating, the right strategy makes all the difference:

Dress in Layers – Use a moisture-wicking base layer to keep sweat off your skin, a warm but breathable mid-layer, and a windproof outer layer if it’s extra cold.
Start Slightly Cold – If you feel perfectly warm at the start, you’re probably overdressed. You should feel a little chilly—your body will warm up fast once you get moving.
Protect Your Hands, Feet, and Face – Gloves, wool socks, and a hat or headband help keep blood flowing to your extremities.
Adjust as You Go – If you start sweating, unzip a jacket or remove a layer before your clothes get soaked.
Hydrate – Cold weather tricks you into thinking you’re not thirsty, but you’re still losing fluids. Drink just like you would in warmer temps.

Cold is Manageable—If You Plan for It

Running in the cold isn’t just about staying warm—it’s about managing heat so you don’t overheat or get too cold. If you dress right, move smart, and listen to your body, winter runs can feel powerful, refreshing, and surprisingly fun. Just don’t forget to warm up your fingers before trying to text when you finish!

Chapter 4: What to Wear and How to Hydrate

When it comes to running in extreme temperatures, your body is already working hard to regulate heat. The right clothing and proper hydration can make or break your run. Wear the wrong gear, and you’ll either overheat or freeze. Get your hydration wrong, and your performance (and safety) takes a hit.

Let’s break down how to dress smart and how to stay hydrated, whether you're sweating in the heat or braving the cold.

Dressing for the Heat: Keep It Light, Keep It Cool

In hot weather, your body is trying to get rid of heat. Your job is to wear clothes that help it do that.

✅ Go for light, breathable fabrics – Look for moisture-wicking materials (like polyester or merino wool) that pull sweat away from your skin. Avoid cotton—it soaks up sweat and holds onto heat.

✅ Light colors are better – Dark fabrics absorb heat, while lighter colors reflect the sun’s rays and keep you cooler.

✅ Loose or fitted? – Slightly loose clothing allows air to circulate, but some runners prefer tighter gear that wicks sweat better. Find what works for you.

✅ Protect your skin – Wear a cap or visor for shade, and don’t forget sunscreen—sweat won’t protect you from UV rays.

🚫 What to avoid: Heavy materials, layers, and anything that traps sweat against your skin.

Dressing for the Cold: Stay Warm, But Don’t Overheat

In cold weather, you need to keep heat in—but not too much, or you’ll end up sweating and getting chilled.

✅ Dress in layers – This is the golden rule:

Base layer: Moisture-wicking (polyester, merino wool) to keep sweat off your skin.
Mid-layer: Insulation (fleece, synthetic) to trap heat.
Outer layer: Windproof or waterproof if needed.

✅ Cover your extremities – Wear gloves, warm socks, and a hat or headband to keep blood circulating to your fingers, toes, and ears.

✅ Zippers and vents are your friend – Jackets with zippers let you adjust as you heat up. Too warm? Unzip. Too cold? Zip back up.

🚫 What to avoid: Cotton (again, it traps moisture), overdressing (if you start sweating, you’ll get cold fast), and super thick socks that make your feet sweat.

Hydration: More Important Than You Think

Most runners know hydration is key in the heat, but it’s just as important in the cold—you just don’t feel as thirsty.

Hydration in the Heat: Drink Early, Drink Often

When running in hot weather, dehydration happens fast. Sweat isn’t just water—it also contains electrolytes like sodium and potassium, which your body needs to function properly.

✅ Drink before you feel thirsty – By the time you’re thirsty, you’re already behind. Try to sip every 15-20 minutes on long runs.

✅ Electrolytes matter – If you’re sweating a lot, plain water isn’t enough. Use electrolyte tablets, sports drinks, or coconut water to replace lost sodium and potassium.

✅ Pre-hydrate – Start drinking water before your run, not just during.

🚫 What to avoid: Drinking too much at once (it can cause stomach cramps), relying only on water without electrolytes, and skipping hydration altogether.

Hydration in the Cold: Don’t Ignore It

You may not feel sweaty in winter, but you’re still losing fluids through breath and sweat. Cold air is dry, and you lose moisture just by breathing.

✅ Sip water regularly – Even if you don’t feel thirsty, keep drinking every 20-30 minutes on long runs.

✅ Warm drinks help – If it’s freezing out, carrying warm tea or an electrolyte drink in an insulated bottle can keep you from getting too cold.

✅ Hydrate post-run too – Cold air dries you out, even if you don’t feel it. Drink water after your run to help recovery.

🚫 What to avoid: Forgetting to drink because you "don’t feel thirsty"—dehydration sneaks up in cold weather.

The Bottom Line

Your body already works hard to regulate temperature—help it out with the right clothing and hydration. Dress in a way that lets your body do its job, and drink enough water to keep everything running smoothly. Whether it’s blazing hot or freezing cold, these small adjustments make a huge difference in how you feel and perform on the run.

Chapter 5: Which is Tougher – Heat or Cold?

Running in extreme heat and extreme cold both push your body to its limits—but in completely different ways. One forces you to dump excess heat, while the other makes you hold onto every bit of warmth. Both can slow you down, drain your energy, and, if you’re not careful, put your health at serious risk. But which one is harder on your system?

Let’s break it down.

1. Energy Use: Who Wins?

🔥 Heat: Your body works overtime to keep you cool. Blood is redirected to your skin, your heart beats faster, and sweating drains your fluids. The result? You burn through energy faster and get tired quickly.

❄️ Cold: Shivering is an energy killer, but once you warm up, your body isn’t working as hard to regulate heat. You don’t lose as much energy to cooling, so as long as you're dressed right, running in the cold can actually feel easier.

Winner: Heat is harder—your body is constantly fighting to cool down.

2. Performance: Which Slows You Down More?

🔥 Heat: When it's hot, your pace drops whether you like it or not. Less blood to your muscles means slower movement, and dehydration makes everything feel harder.

❄️ Cold: Your body feels stiff at first, but once you warm up, running in the cold can feel more efficient. There’s less risk of dehydration, and if you’re dressed right, your body doesn’t have to waste as much energy cooling itself.

Winner: Heat is harder—it’s tough to hit your best pace when your body is struggling to stay cool.

3. Hydration: Which One Dehydrates You More?

🔥 Heat: You sweat a ton, losing both water and electrolytes. If you don’t replace them, dehydration kicks in fast.

❄️ Cold: You still lose fluids through sweat and breathing, but because you don’t feel as thirsty, it’s easier to forget to drink. Dehydration sneaks up on you.

Winner: Heat is worse—dehydration happens faster, and you feel it more.

4. Safety: Which Is More Dangerous?

🔥 Heat Risks:

Heat exhaustion – dizziness, weakness, nausea.
Heat stroke – can be deadly if you don’t cool down.
Dehydration – leads to cramping, confusion, and poor performance.

❄️ Cold Risks:

Frostbite – if your skin stays exposed too long.
Hypothermia – can creep up on you, especially if you're wet.
Slipping on ice – a very real injury risk in winter.

Winner: Tie—Both can be dangerous, but heat-related illnesses escalate faster. Cold weather risks are serious but usually give you more time to react.

5. Mental Toughness: Which One Feels Harder?

🔥 Heat: Running in the heat is mentally brutal—everything feels harder, sweat makes you uncomfortable, and overheating can make you feel lightheaded and sluggish.

❄️ Cold: The hardest part is getting out the door. But once you’re moving, it often feels refreshing and energizing.

Winner: Heat is mentally tougher—it just feels worse as you go.

Final Verdict: Heat is Harder

Both heat and cold come with serious challenges, but heat is tougher on your body overall. It drains your energy, slows your pace, and puts you at higher risk for dehydration and overheating. Cold running has its dangers, but if you layer up and keep moving, your body can usually handle it better.

That said, preference matters—some runners thrive in hot weather, while others love the crisp air of winter runs. The key is knowing how your body responds and preparing accordingly.

Whether you’re sweating under the sun or braving icy winds, the best strategy is always the same: respect the conditions, listen to your body, and run smart.

References:

Sawka, M. N., Leon, L. R., Montain, S. J., & Sonna, L. A. (2011). Exercise under heat stress: Thermoregulation, hydration, acclimation, and nutritional strategies. Physiological Reviews, 91(3), 564–575.
Gavin, T. P. (2003). Clothing and thermoregulation during exercise. Sports Medicine, 33(13), 941–947.
Nuccio, R. P., Barnes, K. A., Carter, J. M., & Baker, L. B. (2017). Hydration and cooling in elite athletes. Sports Medicine, 47(3), 441–456.
O’Brien, C., Young, A. J., & Sawka, M. N. (1999). Thermoregulation during cold exposure: Effects of prior exercise. Journal of Applied Physiology, 87(1), 247–252.
Ihsan, M., Watson, G., & Abbiss, C. R. (2012). The effect of a cold beverage during an exercise session combining endurance and strength training in the heat. Journal of the International Society of Sports Nutrition, 9, 44.
Bongers, C. C. W. G., Thijssen, D. H. J., Veltmeijer, M. T. W., Hopman, M. T. E., & Eijsvogels, T. M. H. (2015). Pre-cooling for endurance exercise performance in the heat: A systematic review. BMC Medicine, 13, 56.
Périard, J. D., Caillaud, C., & Thompson, M. W. (2021). Exercise performance and thermoregulatory responses of elite athletes in hot conditions: A meta-analysis. Sports Medicine, 51(5), 1043–1059.
Iwahashi, M., Chaen, Y., Yanaoka, T., Kurokawa, Y., & Hasegawa, H. (2023). Cold water immersion of the hand and forearm during half-time improves intermittent exercise performance in the heat. Frontiers in Physiology, 14, 1143447.
Gonzalez-Alonso, J., Teller, C., Andersen, S. L., Jensen, F. B., Hyldig, T., & Nielsen, B. (1994). Exercise-generated heat contributes to thermoregulation by increasing cutaneous blood flow. Journal of Experimental Biology, 171, 409–433.
Racinais, S., Alonso, J. M., Coutts, A. J., Flouris, A. D., Girard, O., González-Alonso, J., Hausswirth, C., Jay, O., Lee, J. K. W., Mitchell, N., Nassis, G. P., Nybo, L., Pluim, B. M., Roelands, B., Sawka, M. N., Wingo, J. E., & Périard, J. D. (2015). Consensus recommendations on training and competing in the heat. British Journal of Sports Medicine, 49(18), 1164–1173.