Sauna and Cold Plunge: What the Contrast Does to Your Blood Vessels and Nerves

You are sitting on the top bench. The air burns a little in your nose, the sweat is running, your heart is beating faster than usual. Then you stand up, take a few steps, and in front of you lies the cold plunge pool.

Your feet hesitate. Part of you wants to get in, because everyone says how good it feels. Another part asks quietly: is this actually healthy for me?

Maybe you also know the other side. The moment when your vision goes black after a sauna session. Or the pounding heart in cold water that feels more like an alarm than a refreshment.

That is exactly what this is about. About what alternating heat and cold can do to your blood vessels and your nervous system. And about the question of who it is smart training for, and who it is not.

Wide, narrow, wide: what heat and cold do to your blood vessels

Take a look at your skin after a sauna session. It is flushed, warm, almost glowing. And two minutes after the cold water? Pale and cool, perhaps with a tingling that slowly spreads.

What you are seeing is no coincidence. It is your vascular system at work.

Imagine your skin as a huge radiator with thousands of small valves. When you are too warm, the valves open. Warm blood flows to the surface and gives off heat. When it gets cold, they close, and the blood stays in the core of your body, where your heart, brain and organs need it.

These valves are controlled mainly by your autonomic nervous system. A review by Charkoudian from the Mayo Clinic describes two pathways: nerve fibres that narrow the skin vessels via noradrenaline, and an active system that widens them. According to this review, when the whole body heats up strongly, 80 to 90 percent of the widening of the skin vessels is due to this active system.

Key point: Heat and cold move your skin vessels between two extremes. When the body overheats, skin blood flow can rise to 6 to 8 litres per minute, while local cold can throttle it down to a minimum.

And what happens when cold follows the heat? Here the body of evidence gets thinner, but it stays interesting.

A research group led by Petrofsky from Loma Linda University in California compared contrast foot baths in 14 people with type 2 diabetes and 14 people of the same age without diabetes. In the healthy participants, alternating three minutes of warm with one minute of cold increased skin blood flow more than continuously warm water. In the people with diabetes, this vascular response did not occur.

So vascular training requires blood vessels that can still respond. I will come back to this when we get to the limits.

On your next sauna visit, pay conscious attention to this change in your skin colour. It shows you how flexibly your blood vessels are working right now.

And now you know why your skin glows after the sauna and suddenly turns pale after the plunge pool.

Can blood vessels be trained? What heat alone already shows

Maybe you are wondering: if a single sauna session lowers blood pressure for a while, what happens after weeks and years?

This is where an honest look at the studies pays off. Because almost everything we know about the longer-term benefit comes from heat. Not from the contrast.

You can picture the inner lining of your blood vessels, the endothelium, as a fine sensor. When blood flows more strongly, the endothelium feels the friction against the wall. It responds with messenger substances such as nitric oxide, which relax the vessel muscles. This ability is measured as flow-mediated dilation, FMD for short.

According to a research group from Oregon, heat increases cardiac output and the shear forces on the vessel wall, similar to exercise. For the endothelium, that is a training signal.

Key point: Repeated heat can measurably strengthen the inner lining of your blood vessels. In one study, flow-mediated dilation almost doubled after eight weeks of heat therapy, from 5.6 to 10.9 percent.

A Japanese research group led by Imamura observed something similar with a mild infrared sauna. In 25 men with risk factors such as high blood pressure, raised cholesterol, diabetes or smoking, flow-mediated dilation rose from 4.0 to 5.8 percent after two weeks of daily sauna at 60°C.

And in the long run? The best-known data come from the Finnish Kuopio study. In 2,315 men from eastern Finland, followed for a median of 20.7 years, sauna bathing four to seven times per week was associated with a markedly lower risk of sudden cardiac death than one sauna visit per week, hazard ratio 0.37. How the men cooled down after the sauna was not analysed separately. You can find more on the dose and on the limits of these data in my article Sauna and the heart: how often is healthy?

If you take only one thing from this section: invest first in regular heat that feels good. The cold can come later.

And now you know why I talk about heat first when it comes to vascular training, and only then about ice.

What happens in the first seconds in cold water?

The water reaches your chest. And suddenly your body breathes without asking you. First a deep gasp for air, then quick, shallow breaths. Your heart is racing.

This is not a character trait and not a weakness. It is an ancient reflex.

Researchers call this reflex the cold shock response. Cold receptors in the skin report the sudden drop in temperature, and your nervous system switches to alarm. Breathing, pulse and blood pressure shoot up.

A recent review by Leuci, Tipton and colleagues from Italy, Norway and the United Kingdom describes how this reflex sets in within seconds. In open water, it can become dangerous long before hypothermia is a threat: through inhaled water, through reduced blood flow to the brain when you breathe off carbon dioxide too fast, and through cardiac arrhythmias. In a plunge pool with a solid edge and people nearby, the situation is different. The reflex itself, however, is the same.

Key point: Cold can strongly activate your nervous system. During one hour in water at 14°C, noradrenaline in the blood rose by 530 percent.

This rise also shows up when cold is alternated with the sauna. In a study from Bern, which I discuss in more detail below, immersion in water at 12°C after two sauna sessions raised systolic blood pressure in all groups, in healthy people just as in heart patients.

What is fascinating is the role your mind plays in this. In a study from Portsmouth, before immersion in water at 15°C, the researchers told eleven participants that the water would be five degrees colder this time. But it had stayed just as cold. Even so, their heart rate reached an average of 136 instead of 124 beats per minute, and in a second part of the study, anxiety even slowed down habituation to the cold.

And the sauna beforehand? An older study from the 1970s found that prewarmed skin markedly weakened the hurried breathing in cold water. For this, the participants were warmed in a sauna so that their skin became warm, but their body temperature barely rose. That fits with what many people experience: after the sauna, cold water often feels less brutal. For today's plunge pools, this has not been established.

What does that mean for you in practice? Get in slowly instead of jumping. Keep your head above water. Make each out-breath deliberately long, especially during the first few breaths. And if you are new to this, start with cold water on your legs and arms. I also describe what a gentle start can look like in Ice bathing for beginners.

And now you know why your breath does what it wants at first in cold water, and why that can get easier each time.

Accelerator and brake: how the contrast challenges your nervous system

Do you know this feeling after the third round? First the tingling, then a deep, almost heavy calm. Some people describe it as the clearest moment of the day.

What happens there has to do with the two sides of your autonomic nervous system.

Imagine your nervous system as a car with an accelerator and a brake. The sympathetic nervous system is the accelerator. It speeds up your heart and breathing and sends out noradrenaline. The parasympathetic nervous system, with the vagus nerve as its most important strand, is the brake. It slows your pulse and switches to recovery.

Heat and cold both step on the accelerator, each in its own way. What is interesting is what happens afterwards. One window onto this is heart rate variability, the fine fluctuations between two heartbeats. The more say the vagus has, the larger they become.

Key point: After a cold stimulus, the vagus nerve can take over again more quickly. In a randomised study, a vagal measure of heart rate variability was higher after five minutes in cold water than after recovery in air, 2.32 compared with 1.98.

A Paris study using cold chambers showed something similar. After three minutes of cold, the vagal values of heart rate variability rose markedly, and the pulse fell. Blood pressure only rose when the whole body, head included, was exposed to the cold, and barely at all when the head was spared. The authors conclude that cold over the whole body stimulates the nervous system more strongly than cold on one part of the body.

I describe how you can also engage the brake through your breathing in the article Breathing techniques for the nervous system.

And now you know why this special calm often comes after the contrast, and why your head should stay above water.

Weak circulation: why the contrast needs a careful dose here

Why is the walk from the sauna bench to the pool, of all things, so delicate? Imagine the blood vessels in your skin in the heat as a soaked sponge. A lot of blood collects in the skin. When you stand up, gravity pulls extra blood into your legs. Your heart fills with less blood and pumps less of it towards your brain with each beat.

Key point: Heat reduces your circulation's tolerance for standing upright, and cooling can improve it. Under a simulated standing challenge, the heart's stroke volume fell by 48.7 millilitres in the heat, but only by 10.3 millilitres with cooled skin.

This fits with the fact that blood pressure falls in the sauna. In the Bern study with heart patients, the systolic value reached its lowest point after only six minutes. According to a review, alcohol increases the risk of a drop in blood pressure in the sauna. In my experience, something similar applies to a lack of fluids and to blood pressure lowering medication.

And now you know why the walk from the sauna bench to the pool deserves more attention than the pool itself.

Who is the contrast not for? Clear limits instead of wellness romanticism

Maybe you have read this far and are thinking of your heart. Of your blood pressure. Of a pregnancy. Or of the glass of sparkling wine that seems to belong in the sauna area as a matter of course.

These questions are justified. Because the same physiology that makes the contrast interesting makes it risky for some people.

The contrast brings two strong stimuli in quick succession. In the heat, blood pressure falls and the heart beats faster. In cold water, blood pressure and heart rate rise quickly. A healthy cardiovascular system cushions this. One that is already damaged has less room to manoeuvre.

Key point: Deaths in the sauna are rare and often linked to alcohol. In Finland, between 1990 and 2002, the rate was below 2 per 100,000 inhabitants per year, and half of those who died were under the influence of alcohol.

And how dangerous is the sauna in the first place? Kenttämies and Karkola from the University of Helsinki analysed the deaths that occurred in Finnish saunas between 1990 and 2002. In a country with around 2 million saunas for 5.2 million people, the rate was below 2 per 100,000 inhabitants per year. Half of those who died were under the influence of alcohol. For prevention, the authors recommend less alcohol and not leaving drunk people alone in the sauna.

And now you know why the same cold is training for one person and an unnecessary risk for another.

Contrast baths after exercise: what they can do for recovery

The run was hard, your legs are heavy, and tomorrow brings the next training session. In professional sport, tubs of ice water and pools of warm water often stand side by side after a match. Is that worth it for you too?

Here the research is more extensive than for vascular training. And it has a surprise in store.

Key point: Contrast baths can ease muscle soreness and loss of strength after exercise, but hardly better than other recovery methods. In a meta-analysis of 18 studies, they were superior to passive recovery, but barely to other methods.

A network meta-analysis from China with 57 studies and 1,220 participants compared cold water, contrast baths, warm water and cold chambers directly and indirectly. Contrast baths came out ahead for the muscle enzyme creatine kinase, and cold chambers for muscle soreness and jump performance. According to this analysis, there is no clear winner for everything.

A meta-analysis from Brazil and Australia confirmed the picture. Regular cold of 15°C or less as part of a training programme impaired strength adaptation, with a standardised mean difference of minus 0.60. It had no discernible influence on endurance performance.

You can find more on recovery in Recovery is not doing nothing and in my article on muscle soreness.

And now you know why the same contrast bath can make sense on a competition weekend, but less so for building muscle.

Brown fat, immune system, mood: what else the contrast can do

Maybe you have read that ice bathing burns fat, strengthens your defences and chases away any winter blues. Sounds tempting. But how much of it is backed by evidence?

In 2017, a large review from Portsmouth came to a sober conclusion: the evidence for the many promises around cold water varies greatly in strength, and some of it does not go beyond anecdotes. With that in mind, it is worth looking at the three big promises.

Key point: For metabolism, the immune system and mood, there are interesting indications, but no promises. A 2026 meta-analysis with 13 studies and 4,105 participants could not reliably demonstrate an improvement in mood and stress.

Brown fat: the small furnace in your body

Brown fat is like a small furnace in your body. Unlike white fat, it does not just store energy, but can convert it directly into heat. In adults, it sits mainly in the area around the neck and collarbones.

Immune system: less sickness absence, just as many days of illness

A systematic review from Adelaide with 11 studies and 3,177 participants summarised the effects of cold water in healthy adults. Directly afterwards and one hour later, inflammatory markers rose, an acute stress response. Twelve hours later, stress was lower. Sleep quality and quality of life improved, mood did not.

Mood: lots of hope, thin data

A 2026 meta-analysis from Charité Berlin with 13 studies and 4,105 participants comes to an honest conclusion. For mood, stress and wellbeing, the results mostly pointed in a favourable direction, but they were uncertain and mostly at high risk of bias. In the authors' view, cold water should not currently be regarded as a stand-alone treatment for mental illness.

If you are curious about how cold and your defences interact, read on in Does cold strengthen the immune system? And I describe why heat is more than relaxation in Heat therapy and heat hormesis.

And now you know why I stay curious about brown fat and the immune system, but make no promises.

Kneipp: the contrast on a small scale

Not everyone has a sauna with a plunge pool around the corner. And not everyone wants one. Good thing there is an old tradition that gets by with a garden hose.

Sebastian Kneipp lived from 1821 to 1897 and, from 1855, was the parish priest in Wörishofen, today's Bad Wörishofen, for over 42 years. His teaching rested on five pillars: water, exercise, nutrition, herbs and a balance of body and soul. Unlike many voices in the naturopathy movement of his day, he did not reject vaccinations and medicines, and he sought to work with doctors.

The principle of the contrast pour is simple: first warm, then briefly cold, and cold at the end. Traditionally, the cold pour starts far away from the heart, at the feet or hands, and slowly moves towards the centre of the body. This is clinical tradition, but it fits with what the Paris cold chamber study suggests: the smaller the area, the gentler the stimulus.

I describe in detail how to bring the contrast into your everyday shower in Contrast shower instead of ice barrel and in Cold showers in the morning.

And now you know why a garden hose can sometimes be smarter than an ice barrel.

How to approach the contrast wisely: three levers

Maybe you now feel like trying it. Or you want to rethink your current routine. Both are a good start.

If you are unsure whether your cardiovascular system is ready for the contrast, have a doctor check it beforehand. Especially with pre-existing conditions, a short check with blood pressure, ECG and a conversation is a small investment in many good sauna evenings.

And now you know why a smart contrast has less to do with courage than with a feel for the dose.

Frequently asked questions about sauna and cold plunge

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