Should You Sauna During a Heatwave? What the Science Says!

The UK and France have been under-going a genuine heatwave, with temperatures pushing well over 30°C with some areas in France hitting over 40°C. When the weather gets this hot, most people instinctively avoid saunas altogether. In fact, during heatwaves, saunas across Europe tend to be almost empty, operators often report their quietest weeks of the year.

But should you sauna when it’s already roasting outside? Is it safe? Does it offer any benefit? And how do you hydrate properly if you do decide to go in?

This has got us thinking, and after looking at quite a lot of studies, we’ve put together the grounded, evidence‑based breakdown.

The Core Question: Sauna in Hot Weather, Yes or No?

Short answer: You can sauna safely in hot weather, but you need to adjust your approach. For some people, it’s actually beneficial. For others, it’s better to skip it.

The deciding factors are:

• Medical background (any risk factors?) 
• Your current hydration level
• Your heat tolerance
• Your overall fitness level
• Whether you’ve been in direct sun prior
• Whether you’re doing cold dips afterwards
• How long you plan to stay in the sauna
• Your heat tolerance

Important: Sauna use in hot weather increases cardiovascular strain. A 2018 study in JAMA Internal Medicine found that sauna bathing elevates heart rate to levels similar to moderate exercise. In cooler weather, this is manageable. In a heatwave, the baseline stress is already higher.

If you’re dehydrated, sunburned, or overheated, it’s recommended to take a rain check on the sauna. 

Why Saunas Are Usually Empty During Heatwaves

Operators across the UK, France, and Scandinavia consistently report the same pattern: When outdoor temperatures rise above 25°C, sauna attendance drops sharply. You’ve probably noted this, most commercial gyms with saunas during a heatwave are often empty, or at least the sessions most will endure will be vastly reduced. 

Reasons should be obvious, but include: 

• The body doesn’t crave heat exposure
• Hydration becomes harder to manage
• People feel they’re already “hot enough”

This means if you do go, you’ll likely have the sauna to yourself, a rare luxury…

Potential Benefits of Saunas in Hot Weather

If you’re well hydrated and not heat‑stressed, there are some interesting upsides:

• Improved heat acclimation: Research from the Journal of Applied Physiology shows that controlled heat exposure improves plasma volume and heat tolerance. This is why athletes use saunas during summer training.
• Better sleep: Heat exposure followed by cooling can trigger deeper sleep cycles. A Finnish study from 2020 found sauna users reported improved sleep quality after evening sessions.
• Enhanced circulation: The cardiovascular boost can feel lighter and more pleasant when your body is already warm.

Find a sauna with a plunge pool or cooling facilities, this is really a no brainer, in a heatwave your body needs a genuine cool‑down phase to stay safe. A cold plunge, a cold shower, or even a shaded cool room gives your cardiovascular system the reset it needs between rounds. Without that cooling phase, your core temperature can climb too quickly, especially when the outdoor air is already warm and humid.

When You Should Avoid Saunas in a Heatwave

Skip the sauna if:

• You’ve been in direct sun for more than 30 minute
• You feel lightheaded or sluggish
• You haven’t been hydrating properly throughout the day
• You’re sunburned
• You’ve done intense exercise that day
• You’re already sweating heavily just sitting still

Heat stress compounds quickly, so the positives can quickly be outweighed by the negatives. Sauna should never be layered on top of existing strain.

Hydration: What Actually Works?

This is where people get confused, so let’s make it simple.

Do you need electrolyte sachets?

Not necessarily.

Electrolyte products like Dioralyte or Hydration salts are useful if you’ve lost a lot of fluid, but for most sauna users, they’re overkill.

Is Himalayan salt enough?

Yes, for most people.

A pinch of Himalayan or sea salt in water provides:

• Sodium
• Trace minerals
• Improved absorption

This is perfectly adequate for sauna sessions under 20 minutes.

When electrolyte sachets are useful:

• You’re doing multiple rounds
• You’re combining sauna with cold dips
• You’re sweating heavily before you arrive
• You’re in 30°C+ outdoor temperatures
• You feel slightly depleted or foggy

Electrolyte sachets contain potassium and magnesium, which help prevent cramping and fatigue. They’re not essential, but they’re helpful in heatwaves.