The short answer
In a sauna at 80–100°C your skin heats to about 40°C and your core temperature rises by around a degree. Your heart beats faster, often to 100–150 beats a minute, blood vessels in the skin widen and you sweat, typically around half a kilo. Afterwards, blood pressure falls for a while. Regular use trains the body to cope with heat.
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The first few minutes
Hot air meets your skin, and your skin temperature climbs quickly towards 40°C. Sensors in the skin tell your brain you’re heating up, and it responds by opening up the small blood vessels near the surface so warm blood can shed heat. Your face and body flush.
Five to fifteen minutes in
- You sweat. Sweat glands switch on and sweating increases steadily. Over a session, most people lose around half a kilo, mostly water with some salt.
- Your heart works harder. To keep blood flowing to the widened skin vessels, your heart rate rises, often to 100 to 150 beats a minute, depending on the heat and how used to it you are. The heart pumps more blood each minute, much as it does in moderate exercise.
- Your core temperature rises, more slowly than your skin, typically by around 1°C.
When you come out
Blood vessels stay open for a while, and blood pressure falls. In a Finnish study of 102 adults, average systolic blood pressure was about 7 mmHg lower straight after a 30-minute session, and still lower after half an hour of rest. Arteries were less stiff too. This is why you might feel light-headed if you stand up quickly, and why alcohol, which widens blood vessels further, is a bad mix with saunas.
A cold plunge or shower does the opposite in the short term: blood vessels in the skin clamp shut, and heart rate and blood pressure spike for a minute or two. See contrast therapy.
What changes with regular use?
Your body adapts to repeated heat, much as it does to training in a hot climate:
- You start sweating sooner and more, and your sweat becomes less salty.
- Your plasma volume, the liquid part of the blood, increases. In six runners who used a sauna after training for three weeks, it rose by about 7%.
- Your heart rate rises less for the same heat.
Researchers think repeated heat stress also trains the lining of blood vessels to work better, which may explain some of the links with heart health and blood pressure.
When does it become dangerous?
When core temperature keeps rising because you stay in too long, or your body can’t shed heat because you’re dehydrated, drunk, unwell or on certain medicines. Signs include a headache, nausea, dizziness, confusion and a racing heart. Leave straight away if you feel any of them. See who should avoid saunas.
Keep reading
- Public sauna etiquette: a guide for UK first-timers
What to wear, what to bring and the unwritten rules of UK public saunas and sauna clubs, from towels and talking to throwing water on the stones.
- Types of sauna: Finnish, infrared, steam room and banya compared
The main kinds of sauna and heat bath compared: temperature, humidity, how long people stay in and what the research covers.
- Infrared vs traditional sauna: which is better?
How infrared saunas differ from traditional Finnish saunas in heat, comfort, cost and evidence, and which suits you.
Common questions
How hot does your body get in a sauna?
Your skin warms to around 40°C within minutes. Your core temperature rises more slowly, usually by about 1°C in a typical session, to around 38°C. Staying in much longer pushes it higher, which is when heat illness becomes a risk.
Is the heart rate rise in a sauna like exercise?
In some ways. Heart rate can reach levels seen in moderate exercise, and the heart pumps more blood each minute. But your muscles aren't working, so it doesn't replace exercise.
Why do I feel dizzy after a sauna?
Because blood vessels in your skin are wide open and blood pressure has dropped. Standing up quickly lets blood pool in your legs. Stand up slowly, sit down if you feel light-headed, and drink some water.
References
- Laukkanen JA, Laukkanen T, Kunutsor SK. Cardiovascular and other health benefits of sauna bathing: a review of the evidence. Mayo Clinic Proceedings, 2018.pubmed.ncbi.nlm.nih.gov
- Hannuksela ML, Ellahham S. Benefits and risks of sauna bathing. American Journal of Medicine, 2001.pubmed.ncbi.nlm.nih.gov
- Heinonen I, Laukkanen JA. Effects of heat and cold on health, with special reference to Finnish sauna bathing. American Journal of Physiology, 2018.pubmed.ncbi.nlm.nih.gov
- Laukkanen T et al. Acute effects of sauna bathing on cardiovascular function. Journal of Human Hypertension, 2018.pubmed.ncbi.nlm.nih.gov
- Scoon GS et al. Effect of post-exercise sauna bathing on the endurance performance of competitive male runners. Journal of Science and Medicine in Sport, 2007.pubmed.ncbi.nlm.nih.gov
First published 4 November 2026. Spotted an error? Tell us and we'll correct it. Read our editorial policy.