Eleven minutes of cold water per week. Fifty-seven minutes of sauna. These are the numbers behind the Søberg Protocol, the most-cited contrast therapy recommendation on the internet. They have been repeated on podcasts, printed on infographics, and built into training programmes as if they were precision doses calibrated in a lab. They weren’t — they are the observed weekly habits of eight experienced Danish winter swimmers whose metabolic profiles were then compared with a control group. Not a prescription or an optimised target, but a record of what seasoned practitioners already did.
Here is what the study actually found, what it didn’t, and how to use it honestly.
What Søberg Actually Studied
Every claim traces to a single paper: a 2021 study published in Cell Reports Medicine led by Dr Susanna Søberg, a Danish metabolic researcher specialising in cold and heat exposure. She compared sixteen men — eight who had been winter swimming regularly for at least two years and eight age- and BMI-matched controls who had not.
By design, the study was observational. Søberg did not assign the swimmers a regimen. She followed them through a winter season, recorded their existing habits, and measured the metabolic differences between the two groups. Already, these men were alternating cold water immersion with sauna sessions two to three times per week. Averaged across the group and the season, their weekly totals came to roughly eleven minutes of cold and fifty-seven minutes of heat.
Those averages became “the protocol.” But Søberg herself is careful about the distinction. In a recent interview with ZOE, she described the numbers plainly: “After a year following them, we saw that they did 11 minutes.” The verb is telling. She saw it. She didn’t prescribe it.
What the team found was significant, but not quite what most summaries suggest. Søberg’s team expected the winter swimmers to have more brown adipose tissue — BAT, the metabolically active fat that burns calories to produce heat — than the controls. They didn’t. What they had instead was dramatically more effective brown fat. During a controlled cooling test, the swimmers’ BAT showed a roughly 3,000% increase in activation compared with approximately 90% in the control group. As a ScienceDaily press release put it: “We expected winter swimmers to have more brown fat than the control subjects, but it turned out that they instead had better thermoregulation.”
Limitations are plain. Sixteen participants, all men, observational design, no ability to establish causation. None of this tells us whether eleven minutes is better than eight, or whether fifty-seven minutes of heat is better than forty. It tells us that experienced practitioners who happened to average those numbers showed markedly better cold-induced thermogenesis than people who didn’t practise at all.
How Observations Became a Prescription
The Søberg study might have stayed a respected niche paper. What turned it into “the protocol” was the podcast of Prof. Andrew Huberman, a Stanford neuroscientist. In a widely viewed Huberman Lab episode, Huberman interviewed Søberg, distilled her findings for a mass audience, and gave the concepts stickier labels. Observational averages became “minimum thresholds.” The end-on-cold principle became “the Søberg Principle.” Huberman himself adopted a structured Tuesday routine built around the numbers and shared it publicly.
Subtle but consequential, that shift changed how millions understood the data. A description of what experienced swimmers already did became an instruction for what listeners should do, and that framing propagated through every blog post and wellness newsletter that followed. If you have been anxious about hitting exactly eleven minutes of cold per week, the anxiety was built on a misreading of what the data supports.
The End-on-Cold Principle
If the weekly totals are guidelines rather than targets, the sequencing principle is the one genuinely prescriptive element. What Søberg calls the Soeberg Principle is straightforward: always end your contrast session on cold, not on heat.
Physiologically, the reasoning is straightforward: when you finish on cold and let your body rewarm itself without assistance — no hot shower, no sauna, no warm towel — you force your brown adipose tissue to activate and generate heat internally, a process called non-shivering thermogenesis. A warm wrap or shower after your final cold exposure short-circuits that window. You feel comfortable faster, but you skip the metabolic work that the Søberg swimmers’ bodies were doing every session.
In practical terms: after your last cold exposure, towel off and get dressed. Accept the shivering. Let your skin warm gradually. No hot shower for at least an hour or two if you want the full thermogenic effect. The discomfort is the mechanism.
A Practical Weekly Breakdown
Weekly totals only make sense when you see the per-session structure. Søberg clarified this in detail, outlining how her winter swimmers actually structured their visits:
Per visit (2-3 times per week):
- Three cold water immersions of 1-2 minutes each
- Two sauna sessions of 10-15 minutes each
- Always ending the visit on cold
Arithmetic follows naturally. Three cold dips averaging 1.5 minutes, done three times a week, gives roughly 13.5 minutes. Two sauna sessions averaging 12 minutes, done three times a week, gives about 72 minutes. Scale down to twice a week and you land closer to 9 minutes of cold and 48 minutes of heat. The “11 and 57” sit comfortably in the middle of this range, which is precisely the point: they are a midpoint, not a cliff edge. If you manage two sessions a week instead of three, or your cold dips run 45 seconds instead of two minutes, you are still operating within the range the data describes.
Søberg’s swimmers used open water in Danish winter, typically 2-6°C (36-43°F), and traditional Finnish-style dry saunas at 80-100°C (176-212°F). If your cold plunge runs warmer, say 10-15°C (50-59°F), slightly longer exposures may feel appropriate. The principle is that the cold should feel challenging by the end of each immersion but not unbearable. Cold showers are a viable alternative — Søberg has said as much — though the physiological stimulus is less intense than full immersion. If cold showers are your only option, they count; you may simply need longer to accumulate meaningful cold stress. As for infrared versus traditional saunas, the study used traditional sauna, and whether infrared produces equivalent thermoregulatory effects is a question the Søberg data cannot answer.
Across our user data from regular contrast therapy practitioners, most accumulate 8-15 minutes of cold per week across two to three sessions, independently converging with the range Søberg observed. That convergence suggests roughly 10-15 minutes per week represents a sustainable equilibrium that experienced practitioners gravitate toward naturally.
Søberg’s own framing reinforces this. “It’s not more is better,” she said in the ZOE interview. “Less is more.”
The Women’s Adjustment
Most coverage of the Søberg Protocol ignores something significant: the original study included only men, and Søberg has since published specific guidance for women that adjusts the numbers.
Women have more active brown adipose tissue than men, which means they may achieve meaningful metabolic responses from shorter cold exposures. Søberg’s recommendations account for menstrual cycle phases: during the follicular phase (the first half of the cycle), cold dips of 1-4 minutes; during the luteal phase (the second half), 1-2 minutes. Weekly totals for women may sit closer to 9 minutes of cold rather than 11.
Shorter exposures for women reinforce what the data has been saying all along. The original numbers were observed in men. Women’s physiology responds differently. What stays the same is the principle: consistent cold exposure, ending on cold. Only the dose shifts.
The Brown Fat Question
Prof. Tim Spector, the epidemiologist and co-founder of ZOE, raised a pointed question during the same podcast in which Søberg discussed her findings. He questioned whether brown fat activation in adult humans can drive the kind of metabolic change that cold therapy advocates suggest. Brown fat, he observed, constitutes “only half a percent or something of our fat” in adults. The rodent studies that first made brown fat famous involved animals with proportionally far more of it.
Spector was not dismissing cold exposure or Søberg’s data. He was asking whether the mechanism people fixate on — BAT activation — is really the primary driver of the benefits practitioners experience. The swimmers in Søberg’s study had clearly superior thermoregulation. Whether brown fat alone explains that, or whether cold adaptation involves a broader set of physiological changes, remains open. The practical guidelines hold. The story about why they work is less settled than most accounts suggest.
The Protocol That Isn’t One
The most useful thing about the Søberg Protocol is the thing almost nobody says about it: it was never designed in a lab. It was found in the wild. Eight men who had been winter swimming for years had independently settled into a pattern — a handful of short cold dips alternated with sauna, two or three times a week, always finishing on cold. Their bodies showed what that pattern produced.
Numbers, metabolic differences, study design — all real, all legitimate within their scope. But the strongest way to use this information is not to treat eleven and fifty-seven as sacred targets. It is to recognise them as the range that experienced practitioners naturally gravitate toward when nobody is telling them what to do. The best protocols look like that. Not optimised in a spreadsheet, but discovered through years of consistent practice by people who kept showing up because it worked.
