Cold vs. Heat: I Read the Studies the Way I Read a Financial Guru’s Pitch

I read health claims the same way I read a financial guru’s pitch. Who was in the sample? What did they actually measure? Compared to what? Most “cold plunges boost dopamine 250%” and “saunas add years to your life” headlines fall apart on one of those three questions. Not because the studies are bad, but because the headline promises something the study never tested.

So I went back to the original papers on cold water and sauna, the ones everybody quotes, and read them the way I’d read a fund’s prospectus. Here’s what they found, what they didn’t, and what I’d actually do with it. I’m not a doctor, and none of this is medical advice. If you have a heart condition, blood pressure issues, or are pregnant, talk to your doctor before trying either one.

The three questions

I wrote about what’s wrong with a lot of financial gurus, and the core problem is the same one that shows up in wellness content: a real result, stretched to fit a bigger promise. My filter is short:

1. Who was studied? A handful of young men in a lab is not you at 45 with a bad back.
2. What was measured? A blood hormone level is not a better decision at 2 p.m.
3. Compared to what? An association in a group of Finnish men is not proof that buying a sauna will do the same for you.

Run the famous studies through that filter and you get a much more useful picture.

The dopamine number

The “250% dopamine” figure comes from a 2000 paper in the European Journal of Applied Physiology by Šrámek and colleagues. Young men sat in water up to their necks for one hour at 32°C, 20°C, and 14°C. At 14°C (about 57°F), noradrenaline in their blood rose about 530% and dopamine about 250%.

Those are big, real numbers. Now the filter. Who: a small group of young men. What: blood hormone levels during a full hour in the water. Compared to: the same men out of the water. Nobody in that study took a two-minute plunge and then did a better job at work. The honest takeaway is that cold water produces a strong wake-up response. Everyone who’s tried it already knew that. The leap to “better focus all day” is the pitch, not the finding.

The cold-shower trial that actually matters

The study I find most useful gets quoted far less. In a Dutch randomized trial published in PLOS One in 2016 (Buijze and colleagues), 3,018 working adults without serious illness were assigned to end their usual warm shower with 30, 60, or 90 seconds of cold every day for a month, or to shower normally.

The cold-shower groups reported 29% less sickness absence from work. Here’s the part people skip: they didn’t report fewer days of being ill. They got sick about as often. They just went in more often anyway. The authors don’t know exactly why, and neither do I, but “felt well enough to show up” is a meaningful result for anyone whose income depends on showing up. It also cost nothing. No tub, no subscription.

The sauna longevity headline

The sauna claim comes from the Kuopio heart study in Finland, published in JAMA Internal Medicine in 2015 (Laukkanen and colleagues). It tracked 2,315 middle-aged men for about 20 years. Men who used a sauna four to seven times a week had about a 63% lower adjusted risk of sudden cardiac death than men who went once a week.

That’s the kind of number that sells saunas. Through the filter: who was studied? Middle-aged men in eastern Finland in the 1980s, where sauna is ordinary. What was measured? How often they said they used a sauna at the start, and what happened to them afterward. Compared to what? Other Finnish men who used saunas less.

That’s an association. Men who can sit in a sauna five times a week may also be healthier, less stressed, or have more free time to begin with. It’s the investing equivalent of noticing that people with big portfolios drive nicer cars and concluding that buying a nicer car builds a portfolio. The researchers adjusted for known risk factors, which helps. It doesn’t turn a correlation into a promise.

The survey problem

You’ll also see “most people sleep better after sauna.” That traces to the Global Sauna Survey (Complementary Therapies in Medicine, 2019): 482 regular sauna users, and 83.5% said sauna helped their sleep. I believe them. But ask 482 people who already love something enough to fill out a 71-question survey about it and you’ll get enthusiasm. Ask Bitcoin holders if Bitcoin is a good idea and you’ll get a similar hit rate.

What the pooled evidence says

The closest thing to an honest scorecard on cold water is a 2025 systematic review and meta-analysis in PLOS One (Cain and colleagues), which pooled 11 studies and 3,177 healthy adults. The findings, roughly:

The authors say plainly that the evidence base is limited: few trials, small samples, not much diversity. That’s the honest state of things in 2026.

One more that cuts against the hype

If you lift, this one matters. A 2015 study in The Journal of Physiology (Roberts and colleagues) had men do 10 minutes of cold water immersion after every strength session for 12 weeks. They gained less strength and muscle than men who did an easy active cool-down instead. Cold right after training seems to blunt some of the adaptation you trained for. I’d keep them apart.

What I’d actually do with all this

Put the evidence together and my rules come out simple:

• Start with the free version. A cold finish to your shower is the best-tested, cheapest option on the list. Do it for a month.
• Use cold for waking up and heat for winding down. Cold is a jolt; most people find heat relaxing. Match the tool to the time of day.
• Don’t buy a $13,000 sauna expecting heart protection. Buy it only if you’d use it several times a week because you enjoy it. A gym with a sauna is a cheap test.
• Keep cold away from lifting if strength matters to you.
• Safety over bravado. Never plunge alone or after drinking, don’t put your head under, hydrate in the heat, and get a doctor’s opinion if anything about your heart or blood pressure isn’t normal.

If you want the practical version, with costs, time per session, and a route-day routine, there’s a write-up of cold plunge vs sauna for focus and recovery for business owners.

The bigger lesson

The same three questions protect your health and your money. Who was in the sample, what was measured, compared to what. A lot of wellness content and a lot of financial content are built the same way: take a real finding, drop the fine print, sell the bigger version. Once you get in the habit of asking, you stop overpaying for both.

I’ll keep doing cold water. I’ve written about how to start taking ice baths safely, and I still stand by starting slow and controlling your breathing. I just do it for what the evidence supports: a hard, short, free-to-cheap reset that helps me feel ready for the day. That’s enough.