Story · Heat and cold

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

Sixteen adults visited a lakeside sauna in Wales on two separate days and sat through two 15-minute rounds at 85°C each time. They rested outside between rounds. On one of the 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 skin-to-core cutaway from the trial's measured averages.

Sweat ≈ 0.00 kg

Core37.35°C

Skin32.0°C

Heart67bpm

Diastolic71mmHg

The clock starts at zero

A rectal probe sets the baseline

After half an hour resting outside, the group's core temperature was close to 37.3°C. Hearts beat about 68 times a minute, and 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 ↗

Round one begins

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 ↗

Seven minutes have gone by

Skin heats first and fastest

Average skin temperature climbs toward 41 or 42°C by the end of the round, and the surface ends up 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 ↗

The round is nearly done

The core barely moves in round one

Core temperature lags well behind the skin. In a 73°C chamber at the University of Iowa, 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 ↗ The lake trial saw it rise 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 ↗

The first round is over

Heart rate is up by about forty beats

Sitting still, the group averaged roughly 110 beats a minute by the end of round one.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. See how that claim scores on our dial.

Everyone leaves the heat

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.

The bathers cool off

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 and kept creeping upward for a while. Core temperature often goes on rising 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 ↗

The heat comes back

The second round starts from a different place

On both days they went back into the same 84.5°C room. After a dip, their bodies carried less stored heat back in, and from here the two days pulled apart.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 sauna part is over

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, hearts ran at 108 and the core reached only 37.55°C. The rise was so small the researchers couldn't tell it apart from zero.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 bear on the researchers' comfort and tolerance scales.

They've rested for five minutes

Lake-day hearts run 18 beats slower

As the group rested outside after the last break, hearts on the lake day beat about 18 times a minute slower on average, about as wide as the difference got all 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 ↗

A quarter hour of rest has passed

Core temperatures hit their widest split of 0.63°C

Core temperature fell faster after the dips, and by now the two days were 0.63°C apart. By minute 80 the lake day's core was back at or just under where it started.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 clock stops at 80 minutes

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. The study couldn't separate that difference 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. Researchers haven't yet tested whether regular dips change what years of sauna use do for health.

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

Unsettled

Heat

“Sauna lowers your blood pressure.”

We built the charts from the paper's own numbers

Every point on the charts is a group average 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. We read the hollow dots off its Figure 2, so treat them as good to within 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 averages. The study didn't measure the ninety seconds inside the lake, so we drew that stretch from the water temperature.