Do Cold Plunges and Saunas Actually Work? What the Evidence Shows

Scroll through any wellness feed and you will eventually hit someone gasping their way into a tub of ice water, or sweating it out in a wood-paneled sauna, promising you everything from better sleep to cured diabetes. The claims are big. The evidence is smaller than the influencers would like you to believe.

What actually happens to your body

In medicine we start with biologic plausibility. Is there a reason to think this could work? Wellness culture tends to stop there. I want to go further and ask whether it does work. But plausibility is the right place to begin.

Cold plunges trigger something called the cold shock response in the first 30 to 90 seconds of submersion in water around 40 to 45 degrees or colder. It is involuntary. You take a deep, gasping breath, about three liters of air, which is roughly triple a normal breath. This is why people who fall through ice often drown. Your sympathetic nervous system floors it. Heart rate climbs, blood pressure spikes, cortisol rises, adrenaline goes up about fivefold, and dopamine roughly doubles. Your superficial blood vessels clamp down to preserve heat for your heart, brain, and kidneys. You may suddenly have to pee.

There is also the mammalian diving reflex, which kicks in when you put your face in cold water. It slows your heart rate. That conflicts with the cold shock response, which is speeding it up. Those dueling signals can precipitate a cardiac arrhythmia, and people have died in the first few minutes of a cold plunge. So do not put your face in the water.

Heat is gentler. Blood vessels in the skin dilate, you sweat, evaporation cools you off. A dry Finnish sauna tolerates higher temperatures than a steam room because sweat can actually evaporate. You can lose liters of water. Heat also releases some endorphins. Uncomfortable, yes. Potentially lethal in the first 90 seconds, no.

Cold plunges for athletes: a dud

The strongest case for cold plunges is in athletic recovery. The theory is tidy. You ice an injury to reduce inflammation, so maybe icing the whole body reduces systemic inflammation and helps you recover.

The data do not really back this up. Cold plunges may modestly reduce subjective muscle soreness. For recovery and actual performance, the evidence is inconsistent and weak. A lot of the studies look at dozens of outcomes and flag the handful that reach significance, which is not how you build a robust result.

One study showed that athletes who did ice baths had smaller spikes in creatine kinase, a marker of muscle damage. But muscle damage is how muscles grow back stronger. Blunting that signal is not obviously a win. If you are a recreational runner training for a 5K, skip the tub. Just exercise.

Heat for athletes, and for the rest of us

Heat has somewhat better evidence in sports, particularly hot tub protocols of about 30 minutes post-workout. If you train in a cold climate and need to compete in a hot one, heat acclimation via sauna or hot tub is legitimately useful. This is a narrow use case for sub-elite and elite athletes.

For regular people, the most cited data come from Finland, which I take with a grain of salt given how invested the country is in its own saunas. A large observational study of Finnish men found that more frequent sauna use was associated with lower cardiovascular mortality. Observational data has well-known limits. People who spend seven days a week in the sauna differ from people who spend two, and reverse causation is a real concern: people with early cardiovascular disease may tolerate saunas less and drop out of the high-use group.

That said, sauna use does raise heart rate and act as a mild cardiovascular stressor, roughly like light exercise. I would rather you walk around the block. But it is not nothing. There is also a small study in women with obesity and PCOS where about an hour a week of hot tub exposure for 8 to 10 weeks produced modest improvements in blood pressure and cholesterol metrics. No weight loss, but changes that looked a little like exercise. For people who cannot or will not exercise, that is at least interesting.

My other suspicion about saunas is less physiologic. Sitting quietly with no phone for 20 minutes is good for you. Whether the heat is doing the work or the quasi-meditative state is doing the work is hard to separate.

The detox claim, and a note on alcohol

You cannot sweat out toxins. Your liver and kidneys are the organs that evolved to detoxify you, and they are extraordinarily good at it. Sweat is 99% water and 1% salt. One study measured organic pollutants in sweat and estimated that an entire day in a sauna, if you somehow survived it, would excrete about 0.01% of your daily accumulated intake. Sweat is for cooling you off. That is what made humans persistence hunters on the African plains.

Alcohol is the one real exception people notice. Alcohol and water move freely together throughout the body, so your sweat alcohol content roughly mirrors your blood alcohol content. You might smell like last night in the sauna. You are not clearing a meaningful amount. Your liver processes about one drink per hour. Sweat clears less than a hundredth of that. The dehydration will likely make your hangover worse.

Who should be careful

Both heat and cold are mild cardiovascular stressors. People with known heart disease or arrhythmia risk should talk to a doctor before doing either. The cold shock response makes cold plunges particularly risky, and the problem with arrhythmias is that most people at risk do not know it until the first episode. Jumping into a cold lake carries extra risk because of drowning. Deaths are well described.

Men trying to conceive should know that prolonged heat exposure reduces sperm counts. Pregnant women should avoid saunas and hot tubs, particularly in the first trimester, because elevated core temperature raises the risk of neural tube defects. Cold plunges during pregnancy appear to carry less evidence of harm, particularly for women who were already doing them.

Frostbite in a backyard tub is unlikely because the water is above freezing, but severe vasoconstriction, including Raynaud's phenomenon, can make fingers, toes, and other exposed parts painfully white and bloodless. Regular cold plungers often wear rubber gloves and booties, and sometimes more, for exactly this reason.

Bottom line

Cold plunges look to me like modern self-flagellation. The evidence for meaningful health benefits is thin, the experience is miserable, and the safety concerns are real. Be kind to yourself and skip it.

Saunas are a different story. The cardiovascular signal is modest and imperfect, but plausible, and the quiet time alone with your thoughts is probably doing real work. If it fits your life, enjoy it. If you are hoping either one will detox you, cure your hangover, or replace exercise, it will not.

I covered this in depth on Wellness, Actually — listen below.

Wellness, Actually Podcast

"What's the deal with cold plunges and saunas?" — Listen to the full episode, including the week's health news and listener Q&A.