Best Time of Day to Run During a Heatwave: How to Stay Safe and Keep Training
Running during a heatwave can feel significantly harder than usual, even at easy paces. Choosing the right time of day can make a big difference in performance, safety, and recovery. With a few smart adjustments, you can still maintain your training without overloading your body.
This page is about staying safe during genuinely extreme, dangerous heat. For picking a normal everyday running routine — morning vs evening, sun exposure and your body clock, outside of a heatwave — see best time of day to run.
| Time window | Heat / UV risk | What it means |
|---|---|---|
| 5–9 AM | Lowest | Coolest air and ground — best window for harder sessions |
| 7–10 PM | Low–moderate | Air cools, but roads and pavements still radiate stored heat |
| 11 AM–5 PM | Highest | Peak air temperature, solar radiation and UV — avoid or go very easy |
Source: Circadian variation in running time-trial performance (Chronobiology International, 2025)
Why Timing Matters in a Heatwave
During a heatwave, both air temperature and humidity stay elevated for longer periods of the day. This increases core body temperature, heart rate, and dehydration risk.
Running at the wrong time can turn an easy session into a high-stress workout, increasing the risk of:
- Heat exhaustion.
- Dehydration.
- Excessive fatigue.
- Poor recovery.
Best Time: Early Morning (5:00–9:00 AM)
Early morning is usually the safest and most effective time to run during a heatwave.
- Lowest daily temperatures.
- Lower UV exposure.
- Reduced humidity in many regions.
- Less environmental heat stored in roads and buildings.
This is the best window for long runs, interval sessions, and steady aerobic work.
Second Best: Late Evening (7:00–10:00 PM)
Evening runs can also be effective, especially if mornings are not possible.
- Temperatures begin to drop after sunset.
- Sun exposure is minimal or absent.
- Lower risk of overheating compared to midday.
However, heat stored in pavement and buildings can keep conditions warmer than expected, especially in cities.
Worst Time: Midday to Late Afternoon (11:00 AM–5:00 PM)
This is typically the most dangerous window to run during a heatwave.
- Peak air temperature.
- Strongest sun intensity.
- Highest surface heat (roads and pavements).
- Increased dehydration risk.
If you must train during this time, reduce intensity significantly or consider indoor alternatives like a treadmill.
How Different Sessions Should Be Timed
| Workout Type | Best Time of Day |
|---|---|
| Long run | Early morning |
| Interval training | Early morning or late evening |
| Tempo run | Early morning preferred |
| Easy run | Early morning or late evening |
| Recovery run | Flexible, but avoid midday |
Extra Factors That Affect Heat Stress
Even within the best time windows, conditions can vary. Pay attention to:
- Humidity levels (high humidity reduces cooling).
- Wind (helps cooling).
- Cloud cover (reduces sun intensity).
- Urban heat retention (cities stay hotter longer).
Training Adjustments During a Heatwave
Even if you run at the optimal time, you may still need to adjust your training:
- Slow your pace and focus on effort or heart rate.
- Reduce interval volume.
- Shorten long runs if conditions are extreme.
- Increase hydration before and after sessions.
Warning Signs You Should Avoid Running
Even early or late runs may be unsafe if you are already overheated or dehydrated.
- Dizziness or light-headedness.
- Headache before starting your run.
- Unusually high resting heart rate.
- Fatigue from previous heat exposure.
In these cases, rest or switch to indoor training.
Frequently Asked Questions
Is early morning always cooler?
Generally yes, but very warm nights can reduce the cooling effect. Still, mornings remain the safest option in most heatwaves.
Can I adapt to running in the heat?
Yes. With gradual exposure over 1–2 weeks, your body becomes more efficient at cooling, but timing your runs correctly is still important.
Is it okay to skip runs during a heatwave?
Yes. Missing or modifying a session during extreme heat is better than risking heat-related illness.
Final Thoughts
The best time to run during a heatwave is early morning, followed by late evening if necessary. Avoid midday heat whenever possible, adjust intensity based on conditions, and prioritise safety over pace. Smart timing allows you to maintain fitness while reducing the risk of overheating and burnout.