Summer arrives and you ditch the long sleeves. You bound out the door at your regular pace, legs feeling light, thinking this is going to be a good one. By kilometre three, everything is harder. Heart rate is ten beats higher than usual. Your legs feel heavy. You finish the run feeling like you ran a tempo when the data says you jogged.

That's not a bad day. That's heat.

The difference between running at 15°C and running at 30°C isn't just discomfort. It's a fundamentally different physiological challenge. Your body is solving a thermoregulation problem that competes directly with the systems you need for performance, and the trade-offs are significant. A comprehensive review by Périard, Racinais, and Sawka (2015) describes this as a competition for limited resources: your cardiovascular system can either cool you or fuel your muscles, but in the heat, it struggles to do both well at the same time.

Understanding what's happening, and adjusting accordingly, is the difference between a productive session and one that buries you for three days.

The physiology: what's actually happening

When your core temperature rises during exercise in the heat, your body redirects blood flow toward the skin to dissipate heat. This is a survival mechanism, and it's non-negotiable. Your body will always prioritise cooling over performance.

The cascading effects are well-documented.

Cardiac drift. With more blood diverted to the skin, less returns to the heart per beat. To maintain cardiac output, heart rate rises, often by 10-20 beats per minute over the course of a session in hot conditions (Coyle & González-Alonso, 2001). You're working harder cardiovascularly to maintain the same pace, and over time, even that compensation isn't enough. Pace drops or effort climbs, usually both.

Elevated RPE. The same pace that felt comfortable at 15°C genuinely is harder at 28°C. This isn't a psychological weakness. Research by Périard and colleagues (2011) demonstrated that perceived exertion rises in heat independently of metabolic cost, meaning your brain is accurately reporting a higher internal load even if your splits look identical. Trusting RPE in the heat isn't softness. It's physiological honesty.

Accelerated glycogen depletion. Febbraio and colleagues (1994) showed that exercising at 40°C versus 20°C increased muscle glycogen use by roughly 40-50%. In practical terms, the fuel tank empties faster in the heat. To compound the problem, your body diverts blood away from the stomach and toward the skin for cooling, which impairs your ability to process nutrition during exercise. The answer isn't to eat more mid-run. It's to reduce intensity so you burn through stores more slowly, and to prioritise proper replenishment after the session.

Reduced running economy. The combined effect of higher heart rate, increased ventilation, greater metabolic cost, and thermoregulatory demand means each kilometre costs more energy. A pace that sits comfortably in Zone 2 at 14°C might sit in Zone 3 at 30°C. The effort is real, even if the pace is identical.

Disrupted sleep and recovery. To fall asleep, your core body temperature needs to drop. In hot weather, especially when overnight temperatures stay high, your body struggles to release heat through the skin. The result is restless, fragmented sleep, exactly when recovery matters most. Poor sleep shows up in suppressed HRV readings the next morning, a clear signal that training may need to be adapted. Heat doesn't just make the session harder. It can compromise the recovery that follows it.

The net result: heat is a hidden training load. Every degree above roughly 15°C adds physiological stress that doesn't show up in your distance or pace, but absolutely shows up in your body's recovery requirements. This is why, as we covered in our piece on training load management, tracking internal load (heart rate, RPE, HRV trends) matters more than external load in summer months.

How much to adjust pace

The most practical guideline, supported by Ely and colleagues' analysis of 1.8 million marathon finishing times across major races (2007), is to expect roughly a 1-2% pace reduction per degree Celsius above 15°C.

In real terms: if your easy pace is 5:00/km at 15°C, expect something closer to 5:30/km at 25°C. That's not a failure. That's the same physiological effort at the correct intensity.

A few nuances worth noting.

Humidity matters as much as temperature. Dry heat allows sweat to evaporate, which is your primary cooling mechanism. Humid heat traps that moisture on the skin, reducing evaporative cooling dramatically. A 28°C day at 40% humidity is a very different proposition from 28°C at 85%.

The best real-time guide is your own body. Heart rate, RPE, and HRV trends will tell you more than any temperature-to-pace chart. If your heart rate is sitting 15 beats above normal at your usual easy pace, the answer is simple: slow down. As we explored in our article on HRV-guided training, a suppressed morning HRV reading after a hot-weather session is a clear signal that the heat imposed more stress than you might have realised.

Don't fight the numbers. Run the effort, not the pace.

Hydration: what the evidence supports

Hydration in the heat is simultaneously simpler and more nuanced than most runners think.

The foundational research (Sawka et al., 2007, ACSM Position Stand on Exercise and Fluid Replacement) is clear: fluid loss of more than 2-3% of body weight begins to measurably impair performance. In hot conditions, sweat rates can reach 1-2 litres per hour depending on intensity, body size, and acclimatisation status. That's a lot of fluid to replace.

The practical approach breaks down into three phases.

Before the run. Pre-hydrate with water and electrolytes in the hours leading up to the session, not in the minutes before. Aim for pale yellow urine colour as a rough marker. Gulping plain water at the door doesn't hydrate tissues. It just sits in the stomach. Adding sodium to your pre-hydration helps your body retain fluid more effectively.

During the run. For sessions over 60 minutes in the heat, carry fluid and drink to thirst. The old advice of drinking on a rigid schedule has largely been replaced by thirst-guided intake, which Cheuvront and Kenefick (2014) found to be a reliable guide for most athletes. Add electrolytes, particularly sodium, for sessions over 75-90 minutes or in high sweat-rate conditions. Sweat isn't just water. Replacing only water without sodium can dilute blood sodium levels, which in extreme cases leads to hyponatremia, a condition that is arguably more common among slower, well-hydrated runners than among dehydrated faster ones.

After the run. Replace roughly 150% of fluid lost during the session over the following two to four hours. Weigh yourself before and after a few hot runs to get a sense of your individual sweat rate. That number is surprisingly consistent and becomes a useful planning tool for the rest of the summer.

Know your sweat rate. The simplest test: weigh yourself in minimal clothing before a 60-minute run, then weigh yourself again immediately after (towel off sweat first). Every kilogram lost is roughly one litre of fluid. Run this test in different conditions, as sweat rate varies with temperature, intensity, and humidity. 

For a more detailed analysis of your sweat composition, including sodium concentration, services like Precision Hydration offer testing that helps you dial in exactly what you're losing and how to replace it.

When heat becomes dangerous

There's an important line between "this is hard" and "this is dangerous," and every runner should know where it sits.

Heat exhaustion presents as heavy sweating, nausea, dizziness, headache, and a rapid, weak pulse. It's the body signalling that it's losing the thermoregulation battle. The response is straightforward: stop running, get to shade or air conditioning, cool down with cold water or ice, and hydrate. Most cases resolve with rest and cooling.

Heat stroke is a medical emergency. Core temperature exceeds 40°C. Confusion, disorientation, slurred speech, loss of coordination, or collapse are the warning signs. Sweating may stop entirely. If you see these signs in yourself or another runner, call emergency services immediately and begin aggressive cooling. The National Athletic Trainers' Association guidelines (Casa et al., 2015) are unambiguous: rapid cooling is the single most important intervention.

The practical rule: if you feel confused, nauseous, or notice your thinking becoming foggy during a hot run, stop. There is no workout worth a heat-related medical event. Walk to shade, cool down, reassess. The session will still be there tomorrow.

Modifying your training for the heat

Accepting the heat and adjusting intelligently is the mark of an experienced runner. Here's what that looks like in practice.

Time of day. This is the single biggest lever you have. Running at 6am versus 2pm in midsummer can mean a 10-15°C difference in temperature and a dramatic reduction in direct sun exposure. If your schedule allows it, early morning is the obvious choice.

Route selection. Shaded routes, coastal or riverside paths with wind exposure, and loops that pass water fountains all reduce heat load. An out-and-back through exposed tarmac at midday is a very different session from the same distance through tree cover at dawn.

Workout structure. Shorten intervals, extend recoveries, and most importantly be flexible. A 5 x 1km session at threshold might become 5 x 800m with longer rest, or you might cap it at 4 reps instead of 5. The goal is the same physiological stimulus at a lower total heat load. To manage core temperature rises, long runs can either be shortened or split in two. For example a 90min run becomes 45 minutes in the morning and 45 minutes in the evening with water breaks built into each.

Reduce the perception of effort. Small changes add up. Wear a hat or visor. Bring a cold towel around your neck. Plan routes around water fountains so you can splash water on yourself. Pick shaded paths over exposed ones. These won't change the thermodynamics, but they meaningfully reduce how hard the effort feels, which helps you complete the session at the right intensity.

RPE over pace. In the heat, pace becomes a less reliable measure of effort. Anchor your sessions more to RPE than heart rate or pace. A "Zone 2 easy run" should feel like Zone 2, whatever pace that requires on the day. What matters is how the effort actually feels, not what the watch says.

Know when to move indoors. When the combination of temperature, humidity, and sun exposure pushes conditions into genuinely dangerous territory, a treadmill is a great option, especially if you want to add intensity. There's no shame in training through tough conditions intelligently. Save the deliberate heat exposure for structured acclimation work, which we will cover in our upcoming piece on heat acclimation for runners.

Training with the heat, not against it

Heat is not an obstacle to be ignored. It's a variable to be managed, like any other part of your training environment. The runners who thrive in summer are the ones who accept the physiological cost, adjust their expectations, and protect the quality of their sessions by being honest about what the conditions demand.

Run the effort. Hydrate intelligently. Know the warning signs. And on the days when the heat wins, let it. There will be cooler mornings ahead (and then you will feel like you are flying).

Next up: Heat Acclimation for Runners: The Complete Guide, where we cover how to systematically build heat tolerance so summer training becomes a strength, not just a challenge to survive.

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