What Hot and Cold Actually Do to You

Contrast Therapy, Part 2 of 4. In Part 1 we traced the history back to Rome and Finland. Now: what actually happens in your body — and an honest look at the science.

Ancient tradition only gets you so far — a lot of old practices didn't survive contact with a lab. Contrast therapy has. Let me show you what the heat and the cold actually do, and I'll be straight with you about where the science is solid and where it's still catching up.

What the Heat Does

The sauna alone is no small thing. The landmark 20-year Finnish sauna study of 2,315 men, led by Dr. Jari Laukkanen, found that men who used a sauna 4–7 times a week had a 50% lower risk of fatal cardiovascular disease and a 40% lower risk of all-cause mortality than men who went once a week.

Here's the mechanism: in the heat, your blood vessels dilate wide open, your heart rate climbs like you're on a light jog, and blood moves out toward your skin. Your body is doing cardiovascular work while you sit still. That's the heat side of the ledger, and it's rock solid.

What the Cold Does

Then you hit the cold, and everything reverses. Vessels constrict, blood rushes back to your core, and your body releases a flood of norepinephrine. That surge is not small — one classic study found cold immersion raised plasma noradrenaline by 530% and dopamine by 250% above baseline.

Norepinephrine is the chemical behind the alertness, the focus, and the mood lift that can last for hours after a plunge. It's one of the largest acute responses you can get from anything that doesn't come in a bottle. This is why people walk away from a cold plunge feeling sharper and lighter, not just colder.

What the Two Together Do

Now cycle between them. Hot, cold, hot, cold. Your blood vessels open and close, open and close. Your circulatory system is doing reps — a pump you can't get from either temperature on its own.

That cycling is the basis for using contrast therapy in recovery. A meta-analysis of contrast water therapy found it significantly reduced muscle soreness and strength loss after exercise compared to just resting. A separate controlled study on delayed-onset muscle soreness found the same. If you train, you've probably felt this without needing a paper to confirm it — it's the same thing I noticed after five weeks away from my sauna left my body sore and stiff. If you're wondering whether to do this before or after a workout, I got into that here.

The Honest Part

Here's what I won't do: pretend there's a mountain of research on the exact hot-cold combination. There isn't yet.

The two halves are well studied on their own. The combination — cycled together as one protocol — is thinner. A 2025 scoping review put it plainly: contrast therapy is already a first-line tool for pain and recovery, people clearly benefit, but the trials are small, the methods vary, and the precise mechanism isn't fully nailed down. The honest summary is: we know it works, we're still working out exactly why.

And there's a good reason the research lags. You can't blind someone to whether they're hot or cold — they always know. Sample sizes stay small. And nobody can patent jumping in cold water after a sauna, so there's no drug-company money funding giant trials. The science lags not because the practice is weak, but because it's free and ancient and hard to monetize in a lab.

The Part No Lab Can Measure

And then there's the thing the studies will never fully capture. You step out of that final round, steam coming off your skin, and your whole body hums. Calm and wide awake at the same time. The Finns don't have a clinical term for it. They just know it's the reason you do the whole cycle and not half of it.

The cold also trains something the heat can't. The sauna asks you to relax — it's one of the last places left to do nothing. The plunge asks you to stay calm when every signal in your body says get out. That carries over. You start responding to hard moments instead of reacting to them.

Frequently Asked Questions

Is contrast therapy scientifically proven? The individual pieces are well supported — sauna heat has strong cardiovascular data, and cold immersion has clear effects on norepinephrine, mood, and recovery. The specific hot-cold combination is real and widely used, but the combined research is still developing, with smaller trials and mechanisms still being studied.

What does contrast therapy do to your body? Heat dilates your blood vessels and raises heart rate; cold constricts them and triggers a large norepinephrine release. Cycling between the two creates a circulatory "pump" linked to faster recovery, reduced soreness, sharper focus, and improved mood.

Why isn't there more research on contrast therapy? It's hard to blind (you always know if you're hot or cold), trials tend to be small, and there's no patentable product to fund large studies. The practice is ancient and free, which makes it undervalued in a research system built around monetizable interventions.

Keep reading:

• Previous → Part 1: Fire and Ice, The Oldest Ritual in the World
• Next → Part 3: From a Horse Trough to Nightjar
• Recovery, felt firsthand → Weeks Away From Sauna
• Timing your sessions → When Is The Right Time To Sauna?
• Start the cold side → Nightjar cold plunges

Ready to feel it yourself? Book a call.

Sources

• Laukkanen et al., Association Between Sauna Bathing and Fatal Cardiovascular and All-Cause Mortality Events, JAMA Internal Medicine, 2015 — PubMed · ACC summary
• Cold water immersion catecholamine response (noradrenaline +530%, dopamine +250%) — FoundMyFitness
• Bieuzen, Bleakley & Costello, Contrast Water Therapy and Exercise Induced Muscle Damage: A Systematic Review and Meta-Analysis, PLOS One, 2013 — PMC
• Vaile et al., The effect of contrast water therapy on symptoms of delayed onset muscle soreness, 2007 — PubMed
• Mechanisms and Efficacy of Contrast Therapy for Musculoskeletal Painful Disease: A Scoping Review, 2025 — PMC