Most people think of the sauna and ice bath combination as two good things done back to back: the cardiovascular benefits of heat plus the anti-inflammatory benefits of cold, neatly stacked. It sounds logical. It also appears to be wrong.
A 2021 study published in Cell Reports Medicine by Dr Susanna Søberg, a metabolism researcher and founder of the Søberg Institute, and colleagues examined male winter swimmers in Denmark who regularly alternated between cold water and sauna. The researchers expected to find that these men had accumulated more brown adipose tissue, the metabolically active fat that burns energy to produce heat. Instead, the winter swimmers didn’t have more brown fat than controls. They had a fundamentally different thermoregulatory system.
What the Søberg study actually found
Søberg’s study compared eight habitual winter swimmers against eight matched controls who did neither. Both groups underwent PET-CT scans and metabolic assessments to measure brown fat volume and cold-induced energy expenditure.
What stood out was the energy cost of staying warm. During controlled cooling, the winter swimmers expended an estimated 500 to 1,000 additional kilocalories over 24 hours compared to controls, roughly equivalent to the caloric output of a long run. Both groups had comparable volumes of brown fat. What differed was how efficiently the winter swimmers’ bodies managed thermal stress: lower resting core temperatures, a recalibrated thermal set point, and a more effective whole-body response to cooling.
“We expected winter swimmers to have more brown fat, but it turned out that they instead had better thermoregulation,” Søberg noted.
A popular narrative has formed around cold exposure and brown fat activation: cold activates brown fat, brown fat burns calories, more cold means more brown fat. Søberg’s data suggests the adaptation is more systemic. Instead of growing a bigger furnace, the body learns to run the entire heating system differently.
The honest caveats
Søberg’s study has real limitations. The sample was small — eight participants per group. All were male. The design was observational, meaning it captured a snapshot of habitual winter swimmers rather than randomly assigning people to the practice and tracking change over time. Self-selection cannot be ruled out: perhaps people whose thermoregulatory systems already work differently are more drawn to cold water swimming. And as Prof. Camilla Scheele of the University of Copenhagen cautioned, higher energy expenditure doesn’t automatically translate to weight loss. If the body compensates with increased appetite, the metabolic advantage narrows.
None of this makes the finding unimportant. It makes it preliminary. Søberg’s study is the best evidence we have that the hot-cold combination produces distinct metabolic effects, and it is a single small study. Both things are true.
Two stressors, two defence systems
Metabolically, Søberg’s data is the most concrete finding. But a second argument is building in the cellular biology.
When the body is exposed to heat stress, it upregulates a family of molecules called heat shock proteins, particularly HSP70 and HSP90. These proteins act as molecular chaperones, stabilising other proteins damaged or misfolded by thermal stress and helping cells repair and survive. Sauna research on heat shock proteins is well established. Regular sauna use triggers a consistent heat shock protein response, and that response appears to be one mechanism behind the cardiovascular benefits observed in Laukkanen and colleagues’ large Finnish cohort studies — including a 63 per cent lower risk of sudden cardiac death among men who used a sauna four to seven times weekly compared with once.
Cold exposure activates a different pathway. A 2022 review in Frontiers in Cell and Developmental Biology detailed the evidence for RBM3, a cold shock protein rapidly upregulated when body temperature drops. RBM3 has shown strong neuroprotective properties in preclinical research, appearing to protect synaptic connections and help neurons survive conditions that would otherwise cause cell death.
A single contrast session could activate both pathways within the same hour. Heat shock proteins repairing cellular infrastructure; cold shock proteins reinforcing neural protection. Two branches of the body’s stress-response system, triggered by two precisely opposed stimuli.
Mark Mattson’s influential 2008 review on hormesis described how exposure to one type of mild stressor can prepare cells to withstand others a phenomenon called cross-modal hormesis. If heat primes cells against oxidative damage and cold primes them against neurodegeneration, the combination may offer a broader adaptive stimulus than either stress alone.
One important qualifier: no study has yet directly measured dual heat shock and cold shock protein activation within the same human contrast therapy session. The individual pathways are well documented. The combined effect in a single session remains theoretical — a strong hypothesis, not a confirmed mechanism.
The recovery question, answered honestly
If the combination triggers unique metabolic and cellular adaptations, does it also recover the body more effectively than cold alone?
Here, the evidence is less clear than most articles admit. A 2013 meta-analysis by Bieuzen and colleagues, pooling data from multiple trials on contrast water therapy, found that alternating between hot and cold water significantly reduced muscle soreness and preserved strength compared to passive recovery. But the same analysis found little meaningful difference between contrast water therapy and cold water immersion alone.
For pure exercise recovery, the addition of heat may not improve much on what cold already provides. This doesn’t undermine the broader case — Søberg’s metabolic data and the dual protein framework are separate arguments — but it does mean the recovery claim should be stated precisely. Contrast therapy recovers well. Whether it recovers better than cold alone is an open question.
For safety, a 2016 study by Radtke and colleagues found the sauna-and-cold-water combination was well tolerated by all 37 participants, including individuals with chronic heart failure, without triggering excessive cardiac stress.
The advantage nobody talks about
The most underappreciated argument for combining sauna and ice bath has nothing to do with brown fat, stress proteins, or muscle soreness. It has to do with whether people keep doing it.
Cold water immersion is powerful. For many people, it is also deeply unpleasant, particularly in the first weeks before the body acclimates and the psychological resistance softens. Drop-off for cold-only practice is steep. People try it, feel the benefits, then find the daily act of getting into cold water too aversive to sustain.
Pairing cold with heat changes this equation. After ten to fifteen minutes in a sauna, the body is vasodilated, core temperature elevated, skin flushed. Stepping into cold water from that state is a different sensory experience than stepping in from room temperature. Contrast is sharper, but the preceding warmth provides momentum. The cold feels purposeful rather than punishing.
Across our installations, we consistently see that users who combine sauna and cold plunge maintain their practice longer than those who do cold therapy alone. This is operational observation rather than controlled trial data, but it addresses a question the formal research hasn’t yet asked: not what the combination does to the body, but whether people stay with it long enough for the body to adapt.
A practice that includes comfort as part of the protocol is more psychologically sustainable than one that consists entirely of endurance. You are not simply tolerating a stressor and recovering from it. You are alternating between two states, each with its own appeal, and the movement between them becomes the practice itself. Physiology only matters if people are still doing it three months from now. The combination is the version most of them are.
Why design makes this work
Contrast therapy’s physiology depends on rapid temperature transition. The vascular response constriction in cold, dilation in heat, requires that the body move between thermal extremes quickly enough for both signals to register within a compressed timeframe. A two-minute walk down a corridor between sauna suite and plunge pool is not trivial. It bleeds heat, breaks momentum, and weakens the cardiovascular stimulus.
Premium facilities build around this understanding. At Rekoop, private suites place the sauna and cold plunge in the same room, within steps of each other. Sessions are structured as 60-minute cycles: three rounds of heat and cold with rest periods between, building toward a cumulative thermal load that no single round could produce. A typical session moves from 15 minutes of sauna to two or three minutes of cold immersion, then rest, then back again — each round deepening the contrast as core temperature rises and the cold phase becomes more acute. At NXT Fit, dual-temperature baths sit directly adjacent to the sauna, allowing transitions in seconds. In both cases, spatial design isn’t aesthetic preference — it is a physiological requirement.
Proximity also shapes psychology. When the cold plunge is visible from inside the sauna, the transition feels like the next movement of a session rather than a separate act of willpower. The practice takes on the structure of rounds and progression, which is part of why, as the adherence data suggests, users return. A well-designed space doesn’t just make contrast therapy possible. It makes the practice feel inevitable.
A different kind of practice
Most people think about sauna and ice bath in additive terms: take the benefits of one, add the benefits of the other, arrive at a longer list. The evidence points somewhere else.
Søberg’s study suggests the combination produces a metabolic adaptation that neither heat nor cold creates in isolation. The dual stress protein framework, though still theoretical, offers a plausible cellular mechanism: two distinct defence pathways activated within a single session. And the adherence data identifies the practical advantage the published literature has overlooked: the combination is the version of thermal therapy that people actually maintain.
These three arguments reinforce each other. A recalibrated thermal set point requires months of consistent exposure; dual stress-response pathways only compound if the sessions keep happening. The combination is sustainable enough to produce the adaptations that justify sustaining it.
Not two therapies done in sequence. A single practice with its own logic, its own evidence, and its own staying power.
