The Vitruvian

The Science of Cold & Heat: Dr. Susanna Søberg on Metabolic Resilience

Dr. Susanna Søberg's research established the minimum cold dose needed to activate brown fat and shift metabolism. Eleven minutes a week, split into short sessions, is enough — if you let the protocol work.

Video·Andrew Huberman·11 min read·June 2026

Dr. Susanna Søberg's research on Copenhagen winter swimmers revealed the minimum dose of deliberate cold exposure needed to shift metabolism and activate brown fat. The protocol is precise, the science is clear, and the benefits accumulate in ways that extend far beyond the plunge.

What the Cold Does to Your Body

The moment skin meets cold water, the body responds at every level simultaneously. Thermoreceptors embedded in the surface tissue fire immediately upon contact, transmitting signals along dedicated nerve pathways to the hypothalamus — the brain's thermoregulatory command centre. The hypothalamus processes the input within seconds and coordinates a full-body defensive response. The singular priority, in that moment, is to preserve core temperature.

Vasoconstriction follows almost instantly, blood vessels in the periphery constricting sharply to pull circulation inward and protect the vital organs. Heart rate rises. Blood pressure spikes. Breathing quickens. The sympathetic nervous system — the body's fight-or-flight architecture — has fully activated, mobilising energy and generating the acute state of alertness that makes cold immersion feel, in its first seconds, like an awakening.

At the same time, the brain and adrenal glands release a surge of catecholamines: dopamine, norepinephrine, and epinephrine. Norepinephrine sharpens focus and sustains attention in a way that persists well beyond the immersion itself; dopamine amplifies motivation and elevates mood, often for several hours after the exposure ends. These neurochemicals are not a side effect of the cold — they are its central mechanism, an evolved response to acute environmental stress that leaves the body more alert and capable than it was before entry.

Cold contact on the face introduces a distinct second response. The trigeminal nerve — activated by cold water touching the forehead, the area around the eyes, and the nose — triggers the diving reflex: a parasympathetic response that slows heart rate and reduces oxygen consumption, anchoring a quality of calm even as the sympathetic system continues to fire. Both systems activate simultaneously, pulling in opposite directions within the same moment.

This simultaneous activation of opposing autonomic systems is what distinguishes cold water immersion from every other recovery modality. No other single practice demands full engagement from both the sympathetic and parasympathetic nervous systems at the same moment and intensity. The body learns to hold tension and composure at once — to remain grounded under conditions that would otherwise override conscious control. That is not metaphor; it is the measurable adaptation that repeated cold exposure builds, session by session.

The practical effect is clarity — a quality of settled attention that arrives during the exposure and often lingers through the rest of the day. A nervous system trained to regulate acute cold stimulus handles the pressures of everyday life with greater precision and less reactivity. Cold immersion does not simply feel invigorating; it reshapes the regulatory circuitry that governs how we respond to challenge.

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Source: Huberman Lab Podcast
Episode: "How to Use Cold & Heat Exposure to Improve Your Health"
Guest: Dr. Susanna Søberg, PhD
Duration: 2 hours 30 minutes
Published: May 15, 2023

Click any timestamp to jump to that moment in the video. Transcript condensed for readability while preserving key insights and scientific detail.

0:00

Andrew Huberman:

Welcome to the Huberman Lab podcast, where we discuss science and science-based tools for everyday life. Today my guest is Dr. Susanna Søberg. Dr. Susanna Søberg completed her doctoral thesis work at the Center of Inflammation and Metabolism and the Center for Physical Activity Research at the University of Copenhagen in Denmark.

Her research has focused on how deliberate cold exposure and deliberate heat exposure can be used to enhance human metabolism. She is the first author of a seminal study which discovered the minimum thresholds for deliberate heat and deliberate cold exposure for increasing brown fat thermogenesis—essentially a mode of increasing heat production and metabolism in the body—and for establishing actionable protocols that can be used outside of the laboratory to improve metabolism and human health.

2:00

Andrew Huberman:

So my first question to get things started is: what is happening when we get into an uncomfortably cold environment? If I'm really hot on a hot day, jumping into a cold pool feels really good. But if I'm already kind of at room temperature, I'm a little bit chilly, getting into that same temperature of water doesn't feel so good. There's a shock there. So if you could just walk us through what happens when we get into uncomfortably cold water—whether it's by way of shower or cold plunge—at the level of our physiology and, if you'd like, our psychology.

3:00

Susanna Søberg:

Thank you for that question. It's really good to just address what actually happens in our physiology when we enter cold water. So when you enter cold water, you have cold receptors in your skin—thermoreceptors—and they are activated immediately. They send signals to your brain, to your hypothalamus, which is your thermoregulatory center.

And what happens is that your body tries to preserve heat in your core. So you get vasoconstriction in your periphery—your blood vessels constrict—so you preserve the heat in your core. At the same time, you get an activation of your sympathetic nervous system, which is your fight-or-flight response.

5:00

Susanna Søberg:

Your heart rate increases, your blood pressure increases, you start breathing faster. You get this release of catecholamines—dopamine, norepinephrine, epinephrine—from your brain and from your adrenal glands. This is the cold shock response, and it's designed to mobilize energy and keep you alert and focused so you can get out of the dangerous situation.

But there's also another response that happens when you submerge into cold water, especially if your face goes under. You activate what's called the diving reflex or the diving response. This is a parasympathetic response—your rest-and-digest system—which actually slows down your heart rate and reduces oxygen consumption. It's an ancient evolutionary mechanism that marine mammals use when they dive deep.

8:00

Andrew Huberman:

So this brings us back to the first question, which is: if I go completely underwater for a moment when I'm in cold water, am I getting a different stimulus than if I'm just neck-deep or chest-deep in cold water?

Susanna Søberg:

Yes, exactly. When your face—especially your forehead and the area around your nose and eyes—contacts cold water, you activate the trigeminal nerve, which triggers this diving reflex. So you get both the sympathetic activation from the cold shock and the parasympathetic activation from the diving reflex. It's this unique combination that makes cold water immersion particularly powerful for training your autonomic nervous system.

10:00

Andrew Huberman:

Fascinating. So we're training opposing systems simultaneously. Now, let's talk about your research. You published this study in Cell Reports Medicine in 2021 that really established the minimum effective dose for cold exposure. Can you walk us through what you found?

12:00

Susanna Søberg:

So we recruited winter swimmers in Copenhagen—people who had been swimming in cold water regularly for two to three seasons. We wanted them to already be adapted but not doing extreme durations. They were doing one to two minutes in the water, two to three times per week.

We measured their brown fat activity using PET-CT scans before and after a period of regular cold exposure. And what we found was that this relatively modest amount of cold exposure—about 11 minutes total per week—was sufficient to significantly increase brown fat activity and metabolic rate.

15:00

Andrew Huberman:

Eleven minutes total per week. Not eleven minutes per session.

Susanna Søberg:

Exactly. Eleven minutes cumulative across the week. And this is key: it was split into multiple sessions. The participants weren't doing one long eleven-minute exposure. They were doing short bursts—one to two minutes at a time—multiple times per week. Because what matters is the temperature change, not the duration of exposure.

18:00

Susanna Søberg:

When you enter cold water, exit, and then enter again, each transition signals your cells to adapt. Your brown fat activates to generate heat. Your blood vessels constrict and dilate. Your nervous system learns to regulate the stress response. This repeated stimulus—this contrast—is what builds metabolic resilience.

If you just stay in the cold for a long time, you're sustaining the stress response, but you're not amplifying the adaptive benefit. In fact, prolonged exposure can lead to hypothermia without giving you additional metabolic benefits.

22:00

Andrew Huberman:

So it's like interval training for metabolism. The recovery between exposures is where the magic happens.

Susanna Søberg:

Precisely. And this is why we call it hormetic stress or micro-stressing the body. You're giving your system a challenge that it can adapt to, rather than overwhelming it. Short, repeated exposures build resilience. Prolonged exposure just builds discomfort.

25:00

Andrew Huberman:

Let's talk about brown fat. For people who aren't familiar, what is brown fat and why does it matter for metabolism?

Susanna Søberg:

Brown adipose tissue—brown fat—is metabolically active tissue that burns calories to generate heat. Unlike white fat, which stores energy, brown fat dissipates energy as heat through a process called non-shivering thermogenesis. It's packed with mitochondria, which is why it looks brown.

Infants have a lot of brown fat because they can't shiver effectively, so they need this tissue to stay warm. Adults have less, but we retain some, primarily around the neck, shoulders, and spine. And the exciting thing is that cold exposure can reactivate dormant brown fat and even convert some white fat into beige fat, which has similar thermogenic properties.

30:00

Andrew Huberman:

So by activating brown fat through cold exposure, you're essentially increasing your metabolic furnace—burning more calories even when you're not in the cold anymore.

Susanna Søberg:

Exactly. The metabolic boost persists. Your resting metabolic rate increases. And this isn't just about calories burned—it's about metabolic health. Brown fat activation improves glucose uptake, improves insulin sensitivity, and has anti-inflammatory effects. It's a powerful tool for metabolic regulation.

40:00

Andrew Huberman:

What about rewarming after cold exposure? I've heard conflicting advice—some people say get in a hot shower immediately, others say let your body rewarm naturally. What does the research say?

Susanna Søberg:

The research is clear: you should allow natural rewarming. Do not jump into a hot shower immediately after cold exposure. The reason is that the metabolic benefit—the brown fat activation—happens during the rewarming phase. Your body is generating heat internally by burning calories. If you force rewarming with external heat, you short-circuit that process.

So after you get out of the cold water, dry off, put on warm clothes if you need to, but let your body do the work of generating heat. That's where the adaptation happens.

50:00

Andrew Huberman:

Let's talk about practical implementation. For someone who's never done deliberate cold exposure before, where should they start?

Susanna Søberg:

Start with cold showers. At the end of your regular shower, turn the water to cold for 30 seconds. Focus on controlling your breathing—that's key. Don't let the cold shock make you gasp uncontrollably. Breathe slowly and steadily.

Do this for a week or two, gradually increasing the duration to one minute, then two minutes. Once you're comfortable with that, you can progress to cold water immersion if you have access to it. But cold showers are sufficient to get the benefits. The principle is the same—you're activating those cold receptors and training your nervous system.

60:00

Andrew Huberman:

What temperature should the water be?

Susanna Søberg:

Cold enough to be uncomfortable but not so cold that you panic. For most people, this is somewhere between 10 to 15 degrees Celsius—about 50 to 60 degrees Fahrenheit. But it's individual. The key is that it should trigger the cold shock response—you should want to get out, but you can control your breathing and stay present.

If you're perfectly comfortable, you're not cold enough. The discomfort is the signal that you're activating the adaptive mechanisms.

70:00

Andrew Huberman:

We've been talking a lot about cold. What about heat? Does adding sauna or heat exposure amplify the benefits?

Susanna Søberg:

Yes, contrast therapy—alternating between heat and cold—can be even more powerful than cold alone. Heat exposure activates heat shock proteins, which are cellular repair mechanisms that enhance resilience to stress. Cold activates cold shock proteins and brown fat thermogenesis. Together, they create a broader adaptive response.

In Scandinavia, the tradition is sauna followed by cold plunge, repeated multiple times. The abrupt transitions train your body to maintain equilibrium under stress. Your cardiovascular system, your nervous system, your cellular repair mechanisms—all of them are being challenged and strengthened.

72:00

Andrew Huberman:

What's the protocol? How hot, how long, how many rounds?

Susanna Søberg:

Sauna at 80 to 100 degrees Celsius—about 175 to 212 Fahrenheit—for 10 to 20 minutes, followed by cold plunge for 1 to 2 minutes. Repeat for 2 to 3 rounds. The key is to end on cold, because that's when you get the brown fat activation and the metabolic boost during rewarming.

And again, allow natural rewarming after the final cold exposure. Don't go back into the sauna to warm up. Let your body generate the heat internally.

80:00

Andrew Huberman:

Let's talk about gender differences. Your study focused on men. Are there differences in how women respond to cold exposure?

Susanna Søberg:

This is a great question, and we chose to study only men in this proof-of-concept study to eliminate confounding variables. But there are known physiological differences. Women have more brown fat than men. They also run peripherally colder—especially in the hands, feet, and ears.

Men are thermocomfortable at about 22 degrees Celsius. Women at 24 degrees. So the thermostat wars of home have been validated by physiology.

81:40

Andrew Huberman:

So women might actually have an advantage when it comes to brown fat activation.

Susanna Søberg:

Potentially, yes. They have more brown fat volume, so theoretically they could achieve similar metabolic benefits with the same—or possibly less—cold exposure. But we need more research specifically on women to confirm this.

What I can say is that women do winter swimming and cold exposure with the same protocols as men, and they report similar benefits in terms of mood, energy, and metabolic improvements. So I think the 11-minute weekly protocol is a good starting point for everyone, regardless of gender.

90:00

Andrew Huberman:

What about safety? Who should be cautious or avoid cold exposure altogether?

Susanna Søberg:

Lean individuals with low body fat are at higher risk for hypothermia because they have less insulation. Cold water saps heat very rapidly, and without sufficient body fat, core temperature can drop to dangerous levels faster than you'd expect.

People with cardiovascular conditions should consult their physician before starting. The cold shock response causes a spike in blood pressure and heart rate, which could be dangerous for someone with heart problems.

And anyone doing cold exposure in natural bodies of water needs to be aware of additional risks—currents, depth, drowning. Always have someone with you. Never do this alone in open water.

100:00

Andrew Huberman:

What are the signs of hypothermia that people should watch for?

Susanna Søberg:

Uncontrollable shivering is the first sign. Then confusion, loss of coordination, slurred speech. If you experience any of these, get out of the water immediately, dry off, and warm up. Hypothermia can be life-threatening.

But if you're following the protocol—short exposures of one to two minutes in water that's uncomfortably cold but not freezing—and you're allowing yourself to shiver afterward to rewarm, you should be fine. The key is gradual adaptation. Don't try to be a hero. Build your tolerance slowly over weeks and months.

120:00

Andrew Huberman:

You mentioned earlier that if someone dreads the cold, they're actually a perfect candidate for cold exposure. Can you explain that?

Susanna Søberg:

Yes. The greater your sympathetic response—the more you dread it, the more uncomfortable it feels—the greater the adaptation. Because you're training your nervous system to regulate stress. If you're perfectly comfortable in the cold, it's harder to get an adaptation response.

It's like strength training. If you can easily lift a weight, you're not going to get stronger. But if the weight is challenging—if it's uncomfortable but manageable—that's where the adaptation happens.

So if you hate the cold, embrace it. That discomfort is the signal that you're getting the benefits.

140:00

Andrew Huberman:

Final question: if someone only has access to cold exposure two days a week instead of three, should they extend the duration of each session to reach the 11-minute weekly total, or keep the sessions short?

Susanna Søberg:

Keep the sessions short. The principle is that temperature change is what matters. So if you can only do two days a week, you could do two rounds per session—get in for one to two minutes, get out and rewarm for a few minutes, then get back in for another one to two minutes.

Multiple transitions in a single session can work. But I wouldn't recommend staying in for five or six minutes straight. That's just sustained stress without the adaptive benefit of the temperature contrast.

150:00

Andrew Huberman:

This has been incredibly informative. Before we wrap up, any final thoughts or advice for people starting their cold exposure journey?

Susanna Søberg:

Start small. Progress gradually. Listen to your body. The goal is not to suffer—it's to adapt. Cold exposure is a tool for building resilience, not a test of endurance.

And remember that the benefits extend far beyond metabolism. People report improved mood, better sleep, increased mental clarity, and a sense of accomplishment. There's something profoundly empowering about voluntarily doing something uncomfortable and learning that you can handle it.

The cold is a teacher. And the lesson is that you're more capable than you think.

150:30

Andrew Huberman:

Dr. Susanna Søberg, thank you so much for being here and for the incredible work you're doing to bring rigorous science to these practices that are becoming increasingly popular. Your research has really established the foundation for how people can safely and effectively use cold exposure.

Susanna Søberg:

Thank you, Andrew. It's been a pleasure.

Note: This transcript has been edited for clarity and readability while preserving the scientific accuracy and key insights from the original 2.5-hour conversation. Full unedited audio available on the Huberman Lab Podcast.

Transcript auto-generated by YouTube. Verbatim — duplicates intentionally preserved.

Eleven Minutes a Week

In 2021, Søberg and her colleagues at the University of Copenhagen published a study in Cell Reports Medicine that established a precise and actionable finding: a minimum effective dose of cold exposure sufficient to produce measurable metabolic benefit. The study recruited Copenhagen winter swimmers — people already adapted to cold water immersion, typically entering for one to two minutes at a time, two to three times per week — and measured brown fat activity using PET-CT scans before and after a structured period of cold exposure.

The result was unambiguous. Approximately eleven cumulative minutes of cold water exposure per week — distributed across multiple short sessions — was sufficient to significantly increase brown fat activity and resting metabolic rate. Not eleven minutes in a single immersion: eleven minutes total, built from entries of one to two minutes each, repeated across the week.

This distinction carries the most important practical insight in the research. What drives metabolic adaptation is not how long any single session runs — it is how many times the body transitions from cold to warmth. Each entry into cold water delivers the stimulus; each exit initiates the adaptive response. The transition itself is the signal, and it reaches maximum intensity at the point of temperature change. Staying submerged for longer adds cumulative stress without proportionally greater benefit.

The interval-training analogy applies with precision. In strength or cardiovascular training, the rest between efforts is where the body rebuilds stronger; the same logic holds for cold exposure. Multiple short immersions create multiple adaptive signals; a single prolonged exposure creates one extended stress event without the recovery intervals that build resilience. Hormesis — the principle of beneficial stress at calibrated doses — requires a challenge that can be repeated and recovered from, not one that simply persists.

Natural rewarming after each session is where the logic completes itself. Brown fat thermogenesis — the metabolic process that converts stored calories into heat — activates during the rewarming phase as the body works to restore core temperature through internal generation. If external heat is applied immediately after cold exposure ends, this process is short-circuited. The body needs to generate its own warmth for the metabolic adaptation to occur.

A hot shower immediately after a cold plunge feels like recovery; it functionally undermines it. The shivers, the deep internal heating, the gradual return of warmth to the extremities — these are not discomforts to minimise. They are the adaptation in progress. Protecting this rewarming window, session after session, is what allows eleven minutes a week to compound into something that meaningfully shifts metabolic function over time.

Short, repeated exposures build resilience. Prolonged exposure just builds discomfort.

Brown Fat and the Case for Contrast

Brown adipose tissue is metabolically unlike any other fat in the body. Where white fat functions as a store of energy, brown fat is a furnace — burning calories as heat through non-shivering thermogenesis. The tissue is dense with mitochondria, which accounts for its colour and its outsized metabolic activity. Brown fat does not accumulate energy; it dissipates it, converting calories into warmth with precision and speed.

Infants carry significant brown fat deposits because they cannot shiver effectively — thermogenesis is their primary mechanism for warmth in the first months of life. Adults retain these deposits primarily at the neck, shoulders, and spine, but in most people they become dormant through years of thermoregulatory comfort. Cold exposure reactivates dormant brown fat. Repeated cold exposure can also induce the conversion of white fat to beige fat, which shares the thermogenic properties of its brown counterpart. The body's metabolic architecture is not fixed; it can be remodelled.

The metabolic benefits extend well beyond calorie burn. Brown fat activation improves insulin sensitivity and enhances glucose uptake from the bloodstream — effects that contribute to long-term energy regulation and metabolic clarity rather than just short-term expenditure. Anti-inflammatory signalling also increases and persists between sessions, accumulating over weeks of regular practice. Brown fat functions as a systemic regulator, and cold exposure is its primary means of activation.

Contrast therapy — alternating sauna and cold water immersion — compounds this adaptive response by layering two distinct stimuli in close succession. Heat exposure at 80 to 100 degrees Celsius activates heat shock proteins: cellular repair mechanisms that build resilience to thermal and oxidative stress, contributing to recovery and long-term vitality. Cold exposure activates cold shock proteins alongside brown fat thermogenesis, elevating metabolic rate and deepening the body's adaptive capacity. Together, they produce a broader signal than either modality delivers alone.

A practical contrast protocol: 10 to 20 minutes in the sauna, followed by 1 to 2 minutes in the cold plunge, for 2 to 3 rounds in total. Always end on cold. The final cold entry, followed by natural rewarming, is where the deepest metabolic adaptation occurs — the body generating its own heat as the protocol's natural conclusion. The sauna prepares the system; the cold delivers the signal; the rewarming integrates the adaptation.

The Scandinavian tradition that shaped this practice understood the rhythm intuitively long before the mechanisms were mapped. The alternation of heat and cold, repeated deliberately across a session, is not passive wellness — it is a precise stimulus that the body is architecturally designed to respond to. Modern research has confirmed what generations of winter swimmers built into ritual: the protocol works because the physiology demands exactly what it provides.

How to Build Your Protocol

The most accessible entry point requires no equipment and no preparation beyond intention. At the end of a regular shower, turn the water to cold and hold it for thirty seconds. The goal in those first sessions is not duration — it is breath control. The cold shock reflex will drive shallow, rapid breathing; counter it with slow, deliberate exhales. Extend the cold finish to one minute, then two, over the course of one to two weeks.

Cold showers deliver the same physiological stimulus as full immersion. The cold receptors activate, catecholamines surge, dopamine elevates — and the nervous system receives the same adaptive signal that a cold plunge provides, concentrated into a controlled interval that builds alertness and mood even at shower depth. For most people beginning a practice, this is sufficient to drive meaningful adaptation. The progression to a cold plunge or open water comes naturally once breath control is established and the initial shock response no longer overwhelms.

If you're perfectly comfortable, you're not cold enough. The discomfort is the signal that you're activating the adaptive mechanisms.

Temperature determines the quality of the stimulus. Target water between 10 and 15 degrees Celsius — roughly 50 to 60 degrees Fahrenheit — uncomfortable enough to trigger the full cold shock response while remaining within a range where breathing stays manageable and presence is possible. If the cold feels tolerable, it is not cold enough. The discomfort is not incidental to the protocol; it is the signal that the adaptive mechanisms are engaged.

A note on biological differences worth incorporating into practice: women carry more brown fat than men on average and tend to run peripherally colder, particularly in the hands, feet, and ears. This creates different sensations at the same water temperature, especially in the first minutes of immersion. The eleven-minute weekly protocol is a sound baseline for all practitioners regardless of gender, but the subjective experience of the same objective stimulus will vary — and that variation is entirely normal.

Safety is non-negotiable. Lean practitioners should start with shorter exposures and progress conservatively, as low body fat means reduced insulation and faster core heat loss. Anyone with cardiovascular conditions should seek physician clearance before beginning, given the significant spikes in blood pressure and heart rate that cold immersion produces. Open water adds risks that a cold plunge or shower does not — currents, depth, disorientation — and should never be approached alone. The exit signals are unambiguous: uncontrollable shivering, confusion, loss of coordination.

A strong aversion to cold is not a reason to avoid the practice — it is evidence that the practice will work. The dread response reflects a healthy and intact sympathetic system, precisely the system being trained. The greater the initial resistance, the stronger the adaptive signal when you enter anyway. Discomfort is the stimulus, not the obstacle. Build tolerance across weeks, not sessions; the protocol rewards patience, and that patience is itself part of the practice.

The cold is a teacher. And the lesson is that you're more capable than you think.

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