Story · Heat and cold

Watch a sauna and a lake dip work on a body, minute by minute.

Sixteen adults sat through two 15-minute rounds at 85°C in a lakeside sauna in Wales, on two separate days. Between rounds they rested outside, and on one of the two days each break began with 90 seconds up to the neck in the lake.Source 1 · Crossover trialThermal and cardiovascular responses to sauna are attenuated by adding cold water immersion to cooling breaksOwen TF, Giles CJ, Leaney SF, et al.. Experimental Physiology, 2026.Read the paper ↗PubMed ↗ Scroll to follow the session.

By Adam BognarSep 24, 20267 min readTurn on sound in the header to hear the heartbeat.

Scroll to start the clock ↓

Round one · 84.5°C00:00

10°20°30°37°42°60°85°

We drew this cutaway from skin to core using the trial's measured means. It's an illustration of those numbers.

Sweat ≈ 0.00 kg

Core37.35°C

Skin32.0°C

Heart67bpm

Diastolic71mmHg

Before round one

A rectal probe sets the baseline

After half an hour of rest outside, the group's core temperature sat near 37.3°C. Their hearts beat about 68 times a minute, and their skin averaged around 32°C.Source 1 · Crossover trialThermal and cardiovascular responses to sauna are attenuated by adding cold water immersion to cooling breaksOwen TF, Giles CJ, Leaney SF, et al.. Experimental Physiology, 2026.Read the paper ↗PubMed ↗

Minute 1

The air is fifty degrees hotter than the bather

The sauna ran at 84.5°C with 16% humidity.Source 1 · Crossover trialThermal and cardiovascular responses to sauna are attenuated by adding cold water immersion to cooling breaksOwen TF, Giles CJ, Leaney SF, et al.. Experimental Physiology, 2026.Read the paper ↗PubMed ↗ On a thermal camera the bather is the coolest thing in the room. Heat pours in from every side, and in air hotter than skin the only way to lose any of it is to evaporate sweat.Source 2 · ReviewHuman Cardiovascular Responses to Passive Heat StressCrandall CG, Wilson TE. Comprehensive Physiology, 2015.Read the paper ↗PubMed ↗

Minute 7

Skin heats first and fastest

Mean skin temperature climbs toward 41 or 42°C by the end of the round, which puts the surface hotter than the core underneath it.Source 1 · Crossover trialThermal and cardiovascular responses to sauna are attenuated by adding cold water immersion to cooling breaksOwen TF, Giles CJ, Leaney SF, et al.. Experimental Physiology, 2026.Read the paper ↗PubMed ↗

Vessels in the skin open wide to shed heat. With more blood pooled near the surface, the heart speeds up to hold blood pressure steady.Source 2 · ReviewHuman Cardiovascular Responses to Passive Heat StressCrandall CG, Wilson TE. Comprehensive Physiology, 2015.Read the paper ↗PubMed ↗

Minute 12

The core barely moves in round one

Core temperature lags well behind the skin. In a University of Iowa chamber at 73°C, rectal temperature stayed flat for about ten minutes before it started to climb.Source 3 · Crossover trialHeat Stress and Cardiovascular, Hormonal, and Heat Shock Proteins in HumansIguchi M, Littmann AE, Chang SH, et al.. Journal of Athletic Training, 2012.Read the paper ↗PubMed ↗ In the lake trial it rose by only about a quarter of a degree across the whole first round.Source 1 · Crossover trialThermal and cardiovascular responses to sauna are attenuated by adding cold water immersion to cooling breaksOwen TF, Giles CJ, Leaney SF, et al.. Experimental Physiology, 2026.Read the paper ↗PubMed ↗

Minute 15

Heart rate is up by about forty beats

The group averaged roughly 110 beats a minute at the end of round one, sitting still.Source 1 · Crossover trialThermal and cardiovascular responses to sauna are attenuated by adding cold water immersion to cooling breaksOwen TF, Giles CJ, Leaney SF, et al.. Experimental Physiology, 2026.Read the paper ↗PubMed ↗ A 2026 Finnish study of 50 people in ordinary ten-minute rounds recorded almost the same climb, from 79 to 121.Source 4 · Clinical trialTemperature and humidity independently influence thermoregulatory responses during Finnish sauna bathingLaatikainen-Raussi I, Mikkola T, Ihalainen JK, et al.. Temperature, 2026.Read the paper ↗PubMed ↗

That rise is why people call a sauna passive cardio. Read how that claim holds up.

The break

Here the two days split

On one visit the group stepped straight out and sat in the shade in 23°C air. The other visit sent them to the lake first, to sink to the neck in 19.9°C water for 90 seconds before they sat down.Source 1 · Crossover trialThermal and cardiovascular responses to sauna are attenuated by adding cold water immersion to cooling breaksOwen TF, Giles CJ, Leaney SF, et al.. Experimental Physiology, 2026.Read the paper ↗PubMed ↗

Flip the switch to follow either day. The one you aren't following stays on the charts as a dashed line.

Five minutes into the break

The lake pulls skin down to about 29°C

Five minutes after the sauna, skin averaged about 29°C on the lake day and 35°C on the air day, and hearts ran slower after the dip.Source 1 · Crossover trialThermal and cardiovascular responses to sauna are attenuated by adding cold water immersion to cooling breaksOwen TF, Giles CJ, Leaney SF, et al.. Experimental Physiology, 2026.Read the paper ↗PubMed ↗

The core hardly noticed either break. It kept creeping upward for a while, as cores tend to do in the minutes after the heat stops.Source 3 · Crossover trialHeat Stress and Cardiovascular, Hormonal, and Heat Shock Proteins in HumansIguchi M, Littmann AE, Chang SH, et al.. Journal of Athletic Training, 2012.Read the paper ↗PubMed ↗

Round two

The second round starts from a different place

Both groups went back into the same 84.5°C room. After a dip the body carried less stored heat in with it, and the two days pulled apart from here.Source 1 · Crossover trialThermal and cardiovascular responses to sauna are attenuated by adding cold water immersion to cooling breaksOwen TF, Giles CJ, Leaney SF, et al.. Experimental Physiology, 2026.Read the paper ↗PubMed ↗

Minute 40

The dip left hearts 17 beats slower

At the end of round two the air day's core had climbed 0.73°C, to 38.03°C, and hearts averaged 125 beats a minute. On the lake day the core sat at 37.55°C, a rise too small to tell apart from zero, with hearts at 108.Source 1 · Crossover trialThermal and cardiovascular responses to sauna are attenuated by adding cold water immersion to cooling breaksOwen TF, Giles CJ, Leaney SF, et al.. Experimental Physiology, 2026.Read the paper ↗PubMed ↗

People also rated that second round nearly one full step easier to tolerate on the researchers' comfort and tolerance scales.

Five minutes into recovery

The heart-rate gap peaks at 18 beats

Sitting outside after the last break, hearts on the lake day ran about 18 beats a minute slower on average, the widest gap in the session.Source 1 · Crossover trialThermal and cardiovascular responses to sauna are attenuated by adding cold water immersion to cooling breaksOwen TF, Giles CJ, Leaney SF, et al.. Experimental Physiology, 2026.Read the paper ↗PubMed ↗

Fifteen minutes into recovery

The core gap peaks at 0.63°C

Core temperature fell faster after the dips. Fifteen minutes into recovery the two days sat 0.63°C apart, and by the end the lake day sat at or just under its starting temperature.Source 1 · Crossover trialThermal and cardiovascular responses to sauna are attenuated by adding cold water immersion to cooling breaksOwen TF, Giles CJ, Leaney SF, et al.. Experimental Physiology, 2026.Read the paper ↗PubMed ↗

Minute 80

One session can't answer the bigger question

Sweat came to 0.72 kg on the air day and 0.56 kg on the lake day, a gap the study couldn't separate from chance.Source 1 · Crossover trialThermal and cardiovascular responses to sauna are attenuated by adding cold water immersion to cooling breaksOwen TF, Giles CJ, Leaney SF, et al.. Experimental Physiology, 2026.Read the paper ↗PubMed ↗

The trial followed one session in sixteen healthy adults who didn't use saunas regularly. A dip made the heat easier to bear and eased the load on the heart for the rest of that session. Whether regular dips change what years of sauna use do for health is a question nobody has tested.

This session touches these claims

62°H

Stretched

Heat

“A sauna session is as good as a workout.”

92°H

Hot air

Cold

“End your sauna session on cold to burn more fat.”

64°H

Stretched

Heat and cold

“Alternating hot and cold works better than either one alone.”

44°H

Early

Heat

“Sauna lowers your blood pressure.”

We built the charts from the paper's own numbers

Every point on the charts is a group mean from Owen and colleagues.Source 1 · Crossover trialThermal and cardiovascular responses to sauna are attenuated by adding cold water immersion to cooling breaksOwen TF, Giles CJ, Leaney SF, et al.. Experimental Physiology, 2026.Read the paper ↗PubMed ↗ Solid dots are values the paper states in its text. Hollow dots were read off its Figure 2, so treat them as good to a few hundredths of a degree for core temperature and a couple of beats for heart rate. Straight lines join the measured points. The thermal view is an illustration driven by the same means, and the ninety seconds inside the lake, which the study didn't measure, are drawn from the water temperature.