I looked at the forecast on Monday morning — 36°C through Thursday, 34°C on Friday — and my first thought was: how many people are going to go for a lunchtime run this week and feel terrible, and not understand why. This is not an article telling you to stay home. It is an attempt to explain what is actually happening in your body, so you can make a smarter decision than I made the first time I trained through a heatwave.
Before 9 AM or after 7 PM. Peak heat 11 AM–5 PM: move indoors.
↓ 25% intensity above 32°C500 ml 2 hrs before. 150–250 ml every 15–20 min during.
+ electrolytes after 60 minUse RPE, not pace. Same effort = 5–10 BPM higher in heat.
Slow down. Not a weakness.15–19°C for deep recovery. Disrupted slow-wave sleep = reduced muscle repair.
Blackout curtains during the day. Fan for airflow. Cool shower before bed.
First, the thing nobody tells you clearly enough
When you exercise, somewhere between 70 and 100 percent of the energy your muscles produce is released as heat. Not as movement, not as strength — as heat. Your body deals with this normally through sweat and increased blood flow to the skin. Both mechanisms rely on the same basic physics: you need to be cooler than the air around you, or at least close, for heat to leave your body efficiently.
In a heatwave, that gap closes. The air is already hot. Your skin is already warm. The system still works — but it works much harder, and it starts losing ground faster than you expect.
The critical number is 39.5°C core temperature. Above that, your cognitive function starts to degrade, your coordination drops, and you are in genuine heat illness territory. The problem is you often feel fine at 39°C. The warning signs come late, and by the time you feel them, you are already in trouble.
The fitness loss from one skipped session is zero. The recovery cost of heat illness is measured in weeks. That is the whole calculation.
Timing: the lever nobody bothers to use
Train before 9 AM or after 7 PM. Avoid the 11-to-5 window. At peak heat, you are not just fighting a higher ambient temperature — you are also absorbing radiant heat from the ground and surrounding surfaces. In a city like Zurich or Geneva, that reflected heat from tarmac and concrete can add several degrees of effective thermal load on top of the air temperature reading.
If the temperature is above 32°C and you cannot move the session indoors, reduce intensity by roughly 25%.
Hydration, and why the standard advice is mostly useless
Drink 500 ml about two hours before you train. During exercise, 150 to 250 ml every 15 to 20 minutes. After about 60 minutes of sustained effort in the heat, plain water starts to become a problem rather than a solution.
Sweat is not water. It contains sodium, potassium, and magnesium. When you replace the fluid without replacing the electrolytes — particularly sodium — you dilute your blood sodium concentration. That condition is called hyponatremia, and it can cause nausea, confusion, and in serious cases, seizures.
Intensity and the ego problem
In heat, you have to slow down. Not slightly. Meaningfully.
Your heart rate at the same pace or power output will be 5 to 10 BPM higher in hot conditions. The right tool for heat training is RPE — rate of perceived exertion — not your watch. A pace that felt like a 7 out of 10 in April might be a genuine 9 out of 10 in July at noon.
Acclimatisation: the inconvenient truth about time
Your body can adapt to heat. Plasma volume expands, sweat onset gets earlier, cardiovascular strain decreases. After 10 to 14 days of consistent heat exposure, you genuinely perform better in hot conditions.
Days 1 through 5 are the highest-risk window. Treat this first week as an adaptation phase. Shorter sessions, lower intensity, more recovery time.
The sleep issue, which most people completely underestimate
Your bedroom at 11 PM on a heatwave night is probably between 24 and 27°C. Research puts the optimal sleeping temperature for athletic recovery at 15 to 19°C.
Slow-wave sleep — the stage where most muscular repair and hormonal recovery actually happens — is temperature-sensitive. A hot bedroom compresses the deep recovery stages.
Practical steps: blackout curtains during the day, a fan for airflow, a cool shower immediately before bed.
When to stop, and what to look for
Heat stroke is a medical emergency. The key signs are confusion or disorientation, skin that is hot and dry rather than sweaty, and heart rate that does not come down with rest. If you see these — call emergency services.
So what do you actually do this week
Train early or train late. Move intensity down a meaningful notch, and use RPE not pace. Drink before you are thirsty, and add electrolytes beyond an hour. Accept that your times and outputs will be lower — that is correct, not a failure.
Based on peer-reviewed sports physiology research. Not a substitute for medical advice.
