Hot Tub vs Sauna: What the Research Shows
Key insights
- In the first direct head-to-head trial, 45 minutes of hot water immersion at 40.5°C raised core temperature by roughly 1.1°C, against about 0.4°C for a traditional 80°C sauna and essentially no rise at all for a far-infrared cabin 1.
- Only the hot water immersion arm produced a measurable immune response — shifts in circulating immune cell populations and inflammatory cytokines — which the investigators read as the signature of a genuinely meaningful thermal stimulus 1.
- The reason is physics rather than magic: water conducts heat far more efficiently than air, and immersion switches off evaporative sweat cooling, so a 40°C bath loads the body harder than a sauna at twice that air temperature 1 2.
- Ten hot water immersion sessions over two weeks lowered fasting glucose and insulin in sedentary, overweight men, and a single session raised plasma interleukin-6 and nitrite, a marker of nitric oxide availability 2.
- The sauna still holds the decisive advantage on long-term outcomes: two decades of Finnish cohort data tie frequent sauna bathing to lower cardiovascular and all-cause mortality, and no comparable outcome dataset exists for hot tubs 3 4.
For years the hot tub sat in a different cultural category from the sauna. One was a wellness practice with a Finnish pedigree and a growing epidemiological literature behind it; the other was patio furniture that bubbled. That distinction is starting to look less defensible, because in 2025 a group at the University of Oregon did the obvious experiment nobody had run properly: put the same people through a hot bath, a traditional sauna and an infrared sauna, and measure what actually happens.
The result was not close. On the measures that describe how hard the body is being worked — core temperature, cardiovascular strain, immune signalling — hot water immersion outperformed both saunas, and by a wide margin 1. That finding has been widely reported as "hot tubs beat saunas," which is a fair reading of the acute physiology and a poor reading of the whole evidence base.
The two modalities are answering different questions, and the honest comparison has to hold both at once: what a single session does to you today, and what doing it several times a week for twenty years is associated with. Here is where each one actually stands.
The head-to-head study
Twenty healthy adults, half of them women, averaging 24 years of age, each completed three separate bouts of passive heating at least a week apart: 45 minutes of hot water immersion at 40.5°C, three ten-minute rounds in a traditional sauna at 80°C, and 45 minutes in a far-infrared cabin at 45–65°C. Because it was a within-subject design, each person served as their own control, which strips out most of the variation that makes cross-study comparisons of heat therapy so unsatisfying 1.
Hot water immersion raised core temperature by about 1.1°C. The traditional sauna managed roughly 0.4°C. The infrared cabin barely moved it. Cardiovascular responses tracked the same ordering. And when the investigators looked at blood drawn after each condition, only the immersion arm showed a measurable shift in immune cell populations and inflammatory cytokines — the transient, hormetic inflammatory signal that is thought to drive the adaptations people are chasing when they use heat deliberately 1.
Why water loads the body harder than air
An 80°C sauna sounds far more punishing than a 40°C bath, and for the skin it is. But core temperature is governed by net heat balance, not by the number on the thermostat, and water changes that balance in two decisive ways. First, water conducts heat away from — or into — the body far more efficiently than air does, so the transfer of energy is simply faster. Second, and more importantly, immersion disables the body's primary defence against overheating. In a sauna you sweat, the sweat evaporates, and evaporation carries off a large fraction of the heat you are absorbing. Submerged to the shoulders, sweat has nowhere to go, so the cooling mechanism runs but achieves nothing 1.
Hydrostatic pressure adds a third difference. Water pressing on the limbs and torso pushes blood centrally, raising cardiac filling and stroke volume in a way that dry heat does not replicate. The net effect is that a hot tub is closer to a cardiovascular stimulus and a sauna is closer to a thermal one, which is worth remembering when the question is not "which is better" but "which is safe for this particular person."
What repeated hot water immersion does over weeks
Acute physiology is only interesting if it accumulates into something. On that front the hot water immersion literature is thinner than the sauna literature but not empty. In one controlled study, ten sedentary, overweight men were immersed in 39°C water for an hour, then completed a two-week block of ten immersion sessions. A single session raised plasma interleukin-6 and nitrite — the latter an index of nitric oxide bioavailability, and therefore of vascular function. After the two-week block, fasting glucose and insulin concentrations were lower than in the control condition 2.
Ten men over two weeks is a small study, and the authors framed it as a proof of concept for people who cannot exercise rather than as a substitute for exercise in people who can. But it points in the same direction as the acute data: the thermal dose delivered by immersion is large enough to move metabolic markers that a gentler stimulus tends to leave alone.
Where the sauna evidence is stronger — and where it is weaker
The sauna's advantage is not mechanistic. It is that people have been doing it, consistently and in large numbers, for long enough to study the outcomes that matter. In the Finnish cohort work, sauna frequency is inversely associated with fatal cardiovascular events and all-cause mortality, and the association holds after adjustment for the usual confounders; combining frequent sauna use with good cardiorespiratory fitness is associated with lower risk than either alone 3. The broader review literature adds signals for blood pressure, respiratory disease and dementia 4. There is no equivalent body of work for hot tubs, and there may never be, because no country has made hot tubs a near-universal habit that epidemiologists can track for twenty years.
It is also worth being precise about what the sauna does not do acutely. In a randomised crossover trial of 21 healthy adults averaging 66 years, one or two ten-minute Finnish sauna sessions produced no measurable acute improvement in brachial artery flow-mediated dilation or reactive hyperaemia 5. Sauna benefits, in other words, look like an adaptation that builds over months of repetition rather than something you can detect an hour after a single sit — which is exactly why the mortality data matters more than any single-session measurement. The same caution applies in reverse to the hot tub: a bigger acute response is a plausible reason to expect bigger adaptations, not evidence that they occur.
Choosing between them
If the goal is the largest thermal and cardiovascular stimulus per session, the evidence currently favours hot water immersion, and the practical implication is that a hot tub should be treated as a training input rather than a social amenity — with the same attention to duration, temperature and hydration that a sauna protocol gets. If the goal is the option with the deepest outcome evidence behind it, that is still the sauna. Neither answer is wrong; they simply weight recent mechanism against long-run epidemiology differently.
Three practical caveats. The greater strain of immersion is a benefit for a healthy adult and a genuine risk for someone with cardiovascular disease, orthostatic intolerance, or who is pregnant — check with a clinician first. A hot tub held at true therapeutic temperature is a water-chemistry and maintenance commitment that a sauna is not. And the infrared result in the head-to-head study is a reminder that the label on the cabinet tells you less than the temperature it actually sustains; we have written separately on infrared versus traditional saunas and why that distinction matters.
The Contrast Market Perspective
What the Oregon comparison really demonstrates is that the modality matters less than the dose it delivers, and dose is a function of equipment. A cabin that cannot hold 80°C, or a tub that drifts three degrees over a session, is not running the protocol the research describes — it is running a weaker one, and the difference does not show up until you look for the physiological response and find it missing. That is why we care about heater sizing, insulation and temperature stability more than about which category a product sits in. If you are weighing heat and cold options for your space and want to know what each one would realistically deliver, Schedule a consultation.
References
Every claim above draws on peer-reviewed primary sources, listed in full below.
Footnotes
- Atencio JK, Reed EL, Wiedenfeld Needham K, Lucernoni KM, Comrada LN, Halliwill JR, Minson CT (2025). Comparison of thermoregulatory, cardiovascular, and immune responses to different passive heat therapy modalities. American Journal of Physiology-Regulatory, Integrative and Comparative Physiology. PubMed ↩︎
- Hoekstra SP, Bishop NC, Faulkner SH, Bailey SJ, Leicht CA (2018). Acute and chronic effects of hot water immersion on inflammation and metabolism in sedentary, overweight adults. Journal of Applied Physiology. PubMed ↩︎
- Kunutsor SK, Khan H, Laukkanen T, Laukkanen JA (2018). Joint associations of sauna bathing and cardiorespiratory fitness on cardiovascular and all-cause mortality risk: a long-term prospective cohort study. Annals of Medicine. PubMed ↩︎
- Laukkanen JA, Laukkanen T, Kunutsor SK (2018). Cardiovascular and other health benefits of sauna bathing: a review of the evidence. Mayo Clinic Proceedings. PubMed ↩︎
- Gravel H, Coombs GB, Behzadi P, Marcoux-Clément V, Barry H, Juneau M, Nigam A, Gagnon D (2019). Acute effect of Finnish sauna bathing on brachial artery flow-mediated dilation and reactive hyperemia in healthy middle-aged and older adults. Physiological Reports. PubMed ↩︎