Heat Therapy and Heat Hormesis: Why Sweating Is More Than Relaxation
What the sauna may set in motion inside your cells, why heat shock proteins play a part and where the limits are.
Here, the sauna is seen as wellness. A place to switch off. I also see it as something else: a controlled stress stimulus that your cells can respond to by adapting.
Do you know this moment? You are sitting on the top bench. After eight minutes, you can feel your heart pounding. Your skin is glowing, the sweat is running, and part of you wants to get out. Another part of you stays put.
Right in this tension, something fascinating happens. Your body reads the heat as a threat. And it starts to protect itself. Not only with sweat, but deep inside every single cell.
In this article, we look together at how the sauna affects your body through the lens of Clinical Psychoneuroimmunology (KPNI). We talk about heat shock proteins, about hormesis and about large Finnish cohort studies. And we talk honestly about the limits: heart, blood pressure, pregnancy, alcohol and fluids.
What happens in your body when you go into the sauna?
Imagine your body as a house with a smart air conditioning system. As soon as it gets too warm inside, it opens all the windows. The blood vessels in your skin widen, your heart pumps faster and your sweat glands run at full speed.
The remarkable part: this response partly resembles what happens during moderate exercise. Heart rate up, vessels wide, stress hormones active. Except that you are sitting down.
A team at the University of Iowa led by Masaki Iguchi exposed 25 young, healthy adults to a heat chamber at 73 °C for 30 minutes, and on a different day to a control condition at 26 °C. Core temperature rose by 0.8 °C, heart rate to around 131 beats per minute, and systolic blood pressure fell by 16 mmHg. In the blood, the heat shock protein HSP72 rose by an average of 48.7 percent, noradrenaline by 58 percent and prolactin by 285 percent.
What this means for you: even a single session can set a measurable cellular stress response in motion.
Iguchi M et al. J Athl Train. 2012;47(2):184-90. DOI: 10.4085/1062-6050-47.2.184The sauna is not just a place where you relax. It is a training stimulus that your body takes seriously. Relaxation is more the result than the core of it.
And now you know why you sometimes feel after a sauna session much like you do after an easy run.
Heat shock proteins: your cells' repair crew
Have you ever wondered why an egg turns solid in the pan? Heat changes the shape of proteins. They unfold, clump together and lose their function. The same risk exists in your cells, just on a much smaller scale.
Your cells have their own repair crew for this: the heat shock proteins, or HSPs for short. The technical term is molecular chaperones, meaning companions. They keep other proteins in shape, refold misshapen proteins or tag parts that can no longer be saved for removal.
The trigger
Heat causes individual proteins in the cell to lose their shape. That is the alarm signal.
The switch
A regulatory protein called HSF1 is released, moves into the cell nucleus and switches on the genes for heat shock proteins.
The crew
More HSP70 and its relatives are produced. They stabilize proteins and can dampen inflammatory signaling pathways.
The adaptation
If the stimulus is repeated at a suitable dose, the cell could become more robust against future stress. That is hormesis.
HSP70 is especially interesting for metabolism. A research group at the Baker Heart Research Institute in Melbourne found less HSP72 in the muscle of people with obesity and insulin resistance. In mice, the researchers raised HSP72 in several ways, including with heat. As a result, the animals were protected from diet-induced insulin resistance.
Chung and colleagues showed that raising HSP72 in mice went hand in hand with blocking the inflammatory enzyme JNK. This blockade was closely linked to protection from insulin resistance.
What this means for you: the path from heat through HSP70 to healthier glucose metabolism is biologically plausible. It is well documented in animal models, but not yet conclusively shown in humans.
Chung J et al. Proc Natl Acad Sci U S A. 2008;105(5):1739-44. DOI: 10.1073/pnas.0705799105A Japanese review by Nagai and Kaji summarizes that HSP70 in skeletal muscle is linked to less insulin resistance and less chronic inflammation. It discusses heat as a possible option for people who can barely move because of excess weight or joint problems. According to the authors, whether this translates into a measurable benefit for the heart and blood vessels is still an open question.
Stress is not automatically your enemy. A short, measured stimulus can challenge your cells without overwhelming them. What matters is the dose, not the stress itself.
And now you know why, from a KPNI perspective, heat is more than a pleasant feeling.
Hormesis: why the dose decides between benefit and harm
You may know this from exercise. A training session tires you out at first. Afterwards, your body builds up. Too much training without rest, on the other hand, makes you ill.
This principle is called hormesis. A stimulus that causes harm at a high dose can trigger a protective adaptation at a low dose. A Spanish review by Gálvez and colleagues describes balneotherapy and heat applications in exactly this way: heat can stimulate the heat shock response, and many of the observed effects fit the concept of hormesis.
For the sauna, this means: the heat has to be noticeable, otherwise little happens. But it must not be so long or so intense that your circulation gets out of control. Between these two poles lies your personal window.
And now you know why "the more, the better" is the wrong attitude when it comes to the sauna.
What do the Finnish Kuopio studies show about the sauna?
Finland is a country with around two million saunas. That makes it a good place to observe what happens when people sweat regularly for decades. The Kuopio Ischemic Heart Disease Risk Factor Study is the most important source for this.
Tanjaniina Laukkanen and Jari Laukkanen from the University of Eastern Finland followed 2,315 men aged 42 to 60 for a median of 20.7 years. Compared with one sauna session per week, sauna bathing on 4 to 7 days was associated with a hazard ratio for sudden cardiac death of 0.37. They found similar associations for fatal cardiovascular disease and all-cause mortality.
What this means for you: frequent sauna users lived longer in this cohort. Whether the sauna is the cause, or whether healthier people simply go to the sauna more often, is something an observational study cannot answer.
Laukkanen T et al. JAMA Intern Med. 2015;175(4):542-8. DOI: 10.1001/jamainternmed.2014.8187Further analyses came from the same cohort. With 4 to 7 sauna sessions per week, the hazard ratio was 0.34 for dementia and 0.35 for Alzheimer's disease. In men without high blood pressure, frequent sauna bathing was associated with a hazard ratio of 0.54 for developing high blood pressure. In a mixed group of women and men, the hazard ratio for stroke was 0.39.
The immune lens shows up in the data as well. Kunutsor and colleagues found in 2,269 men that 4 to 7 sauna sessions per week were associated with lower high-sensitivity CRP, even eleven years later. CRP is a marker of silent, low-grade inflammation. This could be one link between the sauna and vascular health.
These numbers are impressive, but they are observations from Finland, mostly in middle-aged men. They show an association. They do not promise you a longer life if you start sweating every day from tomorrow.
A review in Mayo Clinic Proceedings by Laukkanen, Laukkanen and Kunutsor summarizes the evidence. As possible mechanisms, it names better endothelium-dependent vasodilation, less arterial stiffness, modulation of the autonomic nervous system, more favorable blood lipid levels and lower blood pressure. At the same time, it points out open questions and the need for randomized trials.
And now you know how to put the often-quoted sauna numbers into perspective.
Blood vessels and blood pressure: what heat may do at the vessel wall
Your blood vessels are lined with a wafer-thin layer called the endothelium. Think of it as a smart wallpaper. It senses the blood flow and releases messenger substances that widen or narrow the vessel.
When you get hot, more blood flows faster through your vessels. This shear force on the wallpaper is a training stimulus. Something similar happens during exercise.
Vienna Brunt and Christopher Minson from the University of Oregon had 20 young, sedentary people immerse themselves in water at 40.5 °C 4 to 5 times per week for eight weeks, while the control group used thermoneutral water. In the heat group, flow-mediated dilation rose from 5.6 to 10.9 percent, pulse wave velocity fell from 7.1 to 6.1 m/s and mean arterial pressure from 83 to 78 mmHg. Nothing changed in the control group.
What this means for you: repeated heat can measurably influence vascular function, at least in young people. The group was small, and the results need confirmation.
Brunt VE et al. J Physiol. 2016;594(18):5329-42. DOI: 10.1113/JP272453Important for you if you take blood pressure medication: during the heat, your blood pressure drops anyway. Add medication, and you may see black spots when you stand up. Get up slowly and sit down first after your sauna session.
And now you know why heat is sometimes called "passive training" for the blood vessels.
Sauna and metabolism: why a single session is not enough
You may have read that the sauna lowers blood sugar. A closer look is worthwhile here, because the evidence is mixed.
Two controlled studies in people with type 2 diabetes found no improvement after a single session. In Montreal, a hot bath in 13 adults showed no acute change in insulin sensitivity and no rise in HSP70 either. In Maastricht, the rise in blood sugar after an infrared sauna in 12 people was even slightly higher than without heat.
Ely and colleagues from Oregon had women with polycystic ovary syndrome and obesity take 30 one-hour hot water baths over eight to ten weeks. Fasting glucose fell from 105 to 89 mg/dl, the insulin area in the glucose tolerance test became smaller and blood testosterone fell. In the control group, everything stayed the same.
What this means for you: for metabolism, regular repetition over weeks seems to matter more than a single session.
Ely BR et al. Am J Physiol Endocrinol Metab. 2019;317(1):E172-E182. DOI: 10.1152/ajpendo.00549.2018A pilot study at the University of Kansas with 18 older people at metabolic risk found high acceptance, lower blood pressure and better brain blood flow after four weeks of warm baths. Here, too, there was no control group, so the results are preliminary.
The sauna is not a substitute for exercise. It can be a complement, especially for people who find exercise difficult. The strongest case is for combining the two.
And now you know why patience matters more than intensity when it comes to heat.
The four KPNI lenses: how heat speaks to your whole system
In KPNI, we do not look at a single organ. We ask how nerves, the immune system, metabolism and hormones interact. Heat touches all four.
Nervous system
In the heat, noradrenaline rises and the sympathetic nervous system is active. Afterwards, the parasympathetic system often takes over. This alternation between tension and recovery could train the flexibility of the autonomic nervous system.
Immune system
Heat shock proteins can dampen inflammatory switches such as JNK and NF-κB. In the Kuopio cohort, frequent sauna bathing was associated with lower CRP and fewer white blood cells.
Metabolism
HSP70 in muscle is linked to better insulin action and healthier mitochondria. In humans, the first small studies point in this direction.
Hormonal system
Prolactin and noradrenaline rise markedly in the heat. In women with PCOS, testosterone fell after repeated warm baths. What these signals mean for everyday life is still poorly understood.
This is exactly what I find so fascinating about heat therapy. It speaks to your system as a whole. It is not about "sweating out" a symptom. It is about a body that can handle stress better. And with that, more room to move in life.
And now you know why, in KPNI, the sauna counts as a resource and not just a ritual.
Who should not use the sauna? Limits and contraindications
This is where it gets serious. Any stimulus that can have an effect can also cause harm. Heat is no exception.
- Heart: unstable angina, a recent heart attack, severe aortic valve stenosis. With stable coronary heart disease, talk to your cardiologist first.
- Blood pressure: very low blood pressure, a tendency to circulatory collapse, medication that lowers blood pressure strongly.
- Infections: acute infection with fever, because the body is already under heat stress.
- Pregnancy: no long, hot sessions, especially in the first weeks. Always discuss it with your gynecologist.
- Alcohol: avoid it altogether before and during the sauna.
In their review, Hannuksela and Ellahham describe sauna bathing as well tolerated by most healthy adults. They name unstable angina, a recent heart attack and severe aortic stenosis as contraindications. According to the paper, alcohol during the sauna increases the risk of a drop in blood pressure, heart rhythm disturbances and sudden death.
What this means for you: most people tolerate the sauna well. With heart disease, the decision belongs in medical hands.
Hannuksela ML, Ellahham S. Am J Med. 2001;110(2):118-26. DOI: 10.1016/s0002-9343(00)00671-9Alcohol and the sauna: a dangerous combination
A Finnish analysis of deaths in the sauna between 1990 and 2002 shows that such deaths are rare, fewer than 2 per 100,000 inhabitants per year. But half of those who died were under the influence of alcohol. The authors recommend drinking less and never leaving anyone who is drunk alone in the sauna.
Pregnancy: caution with core temperature
A meta-analysis from Toronto covering 15 studies found that a raised maternal body temperature in early pregnancy was associated with roughly twice the risk of neural tube defects, with an odds ratio of 1.92. The studies captured heat from various sources, mainly fever. Even so, this is a clear reason to skip long, hot sauna sessions in the first weeks of pregnancy.
Fluids: what you lose needs to go back in
In the sauna, you lose water and minerals through sweat. Drink a glass of water beforehand and enough afterwards until your urine is light again. Headache, dizziness or strong thirst after the sauna are signals that you are short on fluids.
Knowing the limits does not make the sauna less valuable. It makes it safer. Hormesis only works when the stimulus matches your current condition.
And now you know when the sauna is a resource and when it can become a risk.
What dose has been studied?
I am not giving you a fixed set of instructions here. Heat is individual, and your heart, your blood pressure and your stage of life all play a role. But the studies do offer some orientation.
- Frequency: In the Kuopio cohort, 4 to 7 sauna sessions per week were associated with the most favorable outcomes. Even 2 to 3 per week showed a weaker trend in some analyses.
- Duration: In the cohort, sessions longer than 19 minutes were associated with a lower risk of sudden cardiac death than sessions shorter than 11 minutes.
- Temperature: The Finnish sauna usually runs at 80 to 100 °C with dry air. The intervention studies often used water baths of around 40 to 41 °C over longer periods.
- Repetition: Metabolic and vascular effects appeared after weeks, not after a single session.
Three levers you can use starting today
- Start small and regular. Better a short session twice a week that you tolerate well than once a month to the point of exhaustion. Leave the cabin as soon as you feel unwell.
- Sauna without alcohol, with water. Drink water before and after. Get up slowly and sit down first after your session.
- Clarify your limits. If you have heart disease, take blood pressure medication or are pregnant, talk to your doctor before you start.
And now you know that the right dose is not a secret, but a conversation with your body.
Frequently asked questions about how the sauna affects your body
What does the sauna do to the body?
In the sauna, your core temperature rises by about one degree Celsius, your heart rate climbs, the blood vessels in your skin widen and your blood pressure drops for a short time. In a randomized study, the heat shock protein HSP72 in the blood also rose by around 49 percent after 30 minutes at 73 °C. So your body responds in a way that resembles moderate exercise. Repeated regularly, these short stress signals could trigger adaptations in blood vessels and cells.
What are heat shock proteins and why do they matter?
Heat shock proteins are molecular helpers known as chaperones. They keep other proteins in shape, refold damaged proteins or sort them out. Heat is one of the strongest triggers for their production. In studies, HSP70 has been linked to insulin sensitivity and fewer inflammatory signals. Much of this, however, comes from animal and cell studies and has not yet been conclusively shown in humans.
What does hormesis mean in the context of the sauna?
Hormesis describes the principle that a small, controlled stress stimulus can prompt the body to become more resilient, while too high a dose causes harm. In the sauna, heat is that stimulus. The dose is what matters: short and regular sessions might be beneficial, while sessions that are too long, too hot or combined with alcohol can become dangerous.
How often should you go to the sauna?
In the Finnish Kuopio cohort, 4 to 7 sauna sessions per week were associated with the lowest rates of cardiovascular death and dementia, compared with one session per week. This is an observation, not proof of cause and effect. In everyday life, regularity is probably more important than extremes. If you are new to the sauna, it makes sense to start with one or two short sessions per week.
Does the sauna lower blood pressure?
During and shortly after a sauna session, blood pressure usually drops because the blood vessels widen. In the Kuopio cohort, frequent sauna bathing was associated with roughly half the risk of developing high blood pressure. In a controlled study using hot water baths, mean arterial pressure fell from 83 to 78 mmHg after eight weeks. If you take blood pressure medication, talk to your doctor about using the sauna, because dizziness when standing up may occur more often.
Who should not use the sauna?
Unstable angina, a recent heart attack and severe aortic valve stenosis are considered contraindications. Caution is also needed with acute infections with fever, under the influence of alcohol and with markedly low blood pressure. During pregnancy, especially in the first weeks, you should avoid a strong rise in core temperature and discuss the sauna with your gynecologist.
Can I use the sauna during pregnancy?
An older review describes short sauna sessions as well tolerated in uncomplicated pregnancies of healthy women. At the same time, a meta-analysis shows that a raised maternal body temperature in early pregnancy is associated with roughly twice the risk of neural tube defects. That is why the rule is: no long, hot sessions, especially in the first trimester, and make the decision together with your gynecologist.
Why is alcohol in the sauna dangerous?
Alcohol widens the blood vessels even further, disturbs your perception of temperature and can promote heart rhythm disturbances. In a Finnish analysis of deaths in the sauna, half of those who died were under the influence of alcohol. Deaths in the sauna are rare overall, but alcohol is one of the most important avoidable risk factors.
Does the sauna improve insulin sensitivity?
In two controlled studies in people with type 2 diabetes, a single session showed no acute improvement in insulin sensitivity. In contrast, in a controlled study with 30 hot water baths over eight to ten weeks in women with PCOS and obesity, fasting glucose fell from 105 to 89 mg/dl. This suggests that regular repetition, rather than a single session, could be what counts. The studies are small and need confirmation.
How much should I drink after the sauna?
In a typical sauna session, you lose several hundred milliliters of sweat, depending on duration and temperature. Drink water or unsweetened tea before and after the sauna, and if you sweat heavily and often, also replace minerals through a balanced diet. Dark urine, headache or dizziness after the sauna are signs that you are short on fluids.
How this topic connects with others
Heat is a stimulus that reaches into many other areas: heart, metabolism, inflammation and hormones. Here are the connections I consider most important.
More articles from the Natural Forces Guide
- Sauna and the heart: how often is healthy and when is caution needed?
- Does cold strengthen the immune system? What hormesis means
- Ice bathing for beginners: the right start without overwhelming your body
- Red light therapy: what red and infrared light may do in your cells
- Breathing techniques for the nervous system
- Forest bathing (Shinrin-Yoku): what happens in the body when we go into the forest
- Chronobiology: how morning light shapes your internal clock and your sleep
- How much sun does the body need? Between fear of skin cancer and deficiency
- Vitamin D deficiency: why the winter sun is not enough
- See all articles in the Natural Forces Guide
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The Kuopio cohort data show associations, not a cause-and-effect relationship. The intervention studies on heat therapy are small, and the mechanisms around HSP70 come largely from animal and cell models. This is biologically plausible, but in humans it has not yet been shown with the certainty of large randomized trials. This article does not replace individual medical advice.