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Cold plunge evidence is thin
Saunas
For saunas, one study found that they can reduce anxiety, but that study had no control group.
Cold plunges
For cold plunges, some studies suggest they might lower stress hormones. The effect only showed up 12 hours after the plunge and at none of the other time points the researchers looked at, a pattern that raises suspicion of p-hacking.
What the science backs
Both practices touch on an idea that is backed by science: stressing the body can leave it more relaxed later. The area with by far the most studies on that is exercise.
How does p-hacking produce false findings?
When researchers test many outcomes or time points, some will look significant by chance alone. At the usual 0.05 threshold, about one in 20 comparisons of pure noise will pass. In a 2011 paper, Joseph Simmons, Leif Nelson, and Uri Simonsohn showed that flexible analysis choices could make a study find that listening to a children's song made people younger. They used the demonstration to argue for stricter reporting rules.
Why does cold water stress the body?
Sudden immersion triggers the cold shock response: an involuntary gasp, fast breathing, and a spike in heart rate and blood pressure. Physiologist Mike Tipton at the University of Portsmouth has studied it for decades. The response peaks in the first minute and then fades as breathing settles. Repeated exposure shrinks it, a process called habituation.
Why might a sauna resemble exercise?
Heat makes the heart pump faster to push blood toward the skin and shed warmth through sweat. Jari Laukkanen and colleagues at the University of Eastern Finland have reported that heart rate during a sauna session rises to levels seen in light to moderate exercise. Much of the sauna research there is observational, following large groups of Finnish men over time. Observational studies can show links but cannot prove cause.
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