Walk into almost any gym, spa or wellness space in the UK right now and you’ll find two things: a sauna and a cold plunge pool. The people using them will be full of reasons why you should join them. Better sleep, faster recovery, improved mood, stronger immune system — the list goes on.
But what does the science actually say?
We’ve been listening to the Guardian’s Science Weekly podcast episode on sauna and cold water immersion, presented by Madeleine Finlay with science editor Ian Sample and Dr Heather Massey, Associate Professor at the University of Portsmouth’s Extreme Environments Laboratory. It’s one of the clearest, most honest summaries of the current research we’ve come across — and it’s worth a listen if you want the full picture.
Here’s what we took from it, and what it means in practice.
A traditional Finnish sauna sits between 70 and 100°C. Once you’re in, your skin heats up first, then your core body temperature gradually rises from its normal 37°C to around 39°C.
Your body responds by dilating blood vessels (vasodilation), increasing heart rate, and triggering sweating to cool down. Interestingly, even though your heart is working harder, the dilation of blood vessels typically leads to an overall drop in blood pressure. That slightly dizzy feeling when you step out? That’s why.
Cold water immersion below around 15°C triggers what’s known as the cold shock response: a gasp, rapid breathing, and a sharp spike in heart rate, blood pressure, and stress hormones — adrenaline, noradrenaline and cortisol all surge.
That buzzing feeling you get after? Dr Massey confirms it’s largely down to noradrenaline releasing rapidly and in large quantities. It’s a genuine physiological response, not just in your head.
Here’s where it gets honest — and this is worth paying attention to.
As Ian Sample explains on the podcast, there isn’t yet the kind of gold-standard evidence from large randomised controlled trials that researchers would really want to see. What exists is a collection of smaller observational studies showing correlations with certain health markers, not definitive proof of cause and effect.
That said, here’s what the research points towards:
Sauna:
Cold water immersion:
Alternating hot and cold (contrast therapy): The podcast notes that while contrast therapy is used in sports recovery, the specific evidence for alternating sauna and cold water immersion isn’t particularly strong yet. It’s a promising area, but one that needs more research.
One of the most interesting parts of the podcast is the discussion around why these practices make people feel so good — and how much of it might be psychological rather than purely physiological.
Being in a group, connecting with others, being in nature, going through a shared physical challenge — all of these things have genuine impacts on mental wellbeing. And there’s likely a meaningful placebo effect at play too.
As Ian Sample puts it: if you’re in a sauna or an ice bath, you genuinely can’t do anything else except think, meditate, or talk. It’s enforced relaxation. In a world where most people struggle to switch off, that has real value in itself.
Sauna:
Cold water:
The honest answer is: the research is promising, but still developing. The studies are largely observational. The benefits are correlated, not yet conclusively proven. But the physiological responses are real, the anecdotal evidence is overwhelming, and the psychological benefits of regular heat and cold exposure are hard to argue with.
For most healthy people, sauna and cold plunge are low-risk, high-enjoyment practices worth building into a recovery routine. The Finns have been doing it for centuries, and they seem to be doing fine.
At Heila Wellness, we offer cold plunge tub hire for home recovery, events and corporate wellness days across Oxfordshire, Buckinghamshire and Hertfordshire — including our luxury wood-clad plunge tub. Our cold plunge tubs are also available to buy through the Heila Hytte shop, with delivery and setup included. And yes, sauna hire is coming soon too.
Source: Sauna and cold plunge: where does the evidence stand? — Guardian Science Weekly, 2 April 2026
Presented by Madeleine Finlay. With Ian Sample and Dr Heather Massey, Associate Professor, University of Portsmouth Extreme Environments Laboratory. Produced by Ellie Sands, sound design by Joel Cox, executive producer Ellie Bury.
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