Science Weekly (Guardian) · The Guardian

Sauna and cold plunge: where does the evidence stand?

April 2, 2026·17 min·1 clip

Dr Heather Massey explains why cold water below 15 C triggers a gasp, adrenaline surge and a blood pressure spike.

Science Weekly tackles the popularity of sauna and cold water immersion and asks what the science actually shows. Host Madeline Finlay introduces Ian Sample and expert Dr Heather Massey of the University of Portsmouth's Extreme Environments Laboratory. They set out the temperature ranges: saunas at 70 to 100 Celsius, cold water immersion between 10 and 15 Celsius, colder in polar waters. Ian describes the physiological cascade in a sauna, core temperature rising from 37 to 39 Celsius, vasodilation, sweating, increased heart rate and a net drop in blood pressure. Heather describes the cold shock response triggered below 15 Celsius, including the initial gasp, rapid breathing, rise in heart rate and blood pressure, and hormonal surges of adrenaline, noradrenaline and cortisol. The episode turns to evidence: there are no large randomised controlled trials, but a body of observational work, largely on Finnish men, correlates frequent sauna use with longer life and lower cardiovascular disease. A proposed mechanism is vascular elasticity from repeated vasodilation. For cold water, small studies suggest correlations with improved anxiety and depression, and Heather is running a randomised trial whose results are due later in the year. There is also research into inflammatory markers and the idea of cross-adaptation, that tolerating one stressor may help the body tolerate others, but this remains unproven. Contrast therapy, alternating hot and cold, lacks robust data. Psychological and social factors, group bonding, time in nature, enforced relaxation and placebo, likely contribute to perceived benefits. On dosing, the Finnish data suggest four to seven sauna sessions per week correlate with lower blood pressure, but may simply reflect a calmer lifestyle. For cold water, just four or five immersions cut the cold shock response by about 50 percent, with effects lasting around seven months. The episode ends with safety advice: start with warmer outdoor swims, avoid alcohol, stay hydrated, never swim alone, and check with a GP if you have heart issues.

As heard by us

A steady look at a popular ritual, with the evidence kept in clear focus.

It takes a measured line on saunas and cold water immersion, keeping the claims tied to the evidence rather than the hype. The discussion distinguishes what looks well supported from the bigger promises people often attach to heart health, mood, immunity, and adaptation, and it…

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Why you'd press play

If you want the mixed evidence on sauna and cold-water claims, start here.

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