Essentials: Cold & Heat Exposure for Metabolic Health | Dr. Susanna Søberg

Cold Receptors Activate Brown Fat Via Neural and Hormonal Pathways

When the skin detects drops in temperature, cold receptors send signals rapidly to the hypothalamus—the body's temperature regulation center. This central processing results in a spike in neurotransmitters, including noradrenaline and adrenaline, with a minor contribution from cortisol. Noradrenaline, in particular, acts swiftly to activate brown fat. Brown fat is located mainly around the central nervous system, with a large depot under the clavicular bones close to the skin surface, allowing for rapid and efficient heat production.

There are multiple activation routes: skin cold receptors signal both the hypothalamus—which triggers hormone release—and directly link to brown fat via neural pathways. This anatomical arrangement enables immediate brown fat activation on cold exposure, a trait likely developed as a crucial evolutionary adaptation for thermal regulation and survival. Additionally, facial cold and even isolated exposure, such as submerging a hand in cold water, will similarly activate brown fat.

Heat Generation: Muscle Shivering and Brown Fat Collaboration

Thermogenesis is primarily performed by two tissues: brown fat and muscle. When exposed to cold, brown fat engages first, stabilizing body temperature. If additional heat is required, muscular shivering begins, providing further warming. Both tissues thus cooperate efficiently to prevent hypothermia, with brown fat even activating during muscle shivering to maximize heat output.

Sympathetic Nervous System Activates During Initial Cold Exposure Through Hyperventilation and Increased Catecholamine Release in Cold Shock Response

Cold water immersion launches the cold shock response, marked by strong sympathetic nervous system activation. This state is characterized by immediate hyperventilation—rapid, uncontrollable breathing—and a pronounced catecholamine surge (adrenaline and noradrenaline). The stress response is highlighted by the gasping reflex, particularly noticeable in newcomers to cold exposure. Rapid breathing—the body's quick attempt to adapt—serves as a direct indicator of the sympathetic response.

Cold Adaptation Reduces Hyperventilation and Stress Hormones, Maintaining Metabolic Benefits of Brown Fat Activation

With repeated exposure to cold, the intensity of the hyperventilating response and catecholamine surge decreases as the body adapts. Individuals who train with regular cold exposure build resilience, resulting in a more controlled breathing pattern, diminished stress responses, and continued metabolic activation of brown fat. The adaptation strengthens both physical and mental resilience to stress.

Cold Exposure Builds Resilience By Training Stress Response

The repeated stress of cold exposure trains the body to moderate its stress responses. Over time, the initial "shock" phase diminishes, making adaptation easier and allowing individuals to perform better under thermal and psychological stress.

Parasympathetic Activation: Shift From Sympathetic Response to Recovery Mode After 1-2 Minutes of Cold Water Immersion

Within one to two minutes of cold exposure, the body begins to activate the parasympathetic nervous system—the "rest and digest" mode. This transition initiates a recovery state even as some sympathetic activity continues.

Submersion Triggers a Diving Response, Slowing Oxygen Consumption and Preserving Energy

Full-body or facial submersion in cold water invokes the diving response, where oxygen consumption slows dramatically to preserve internal energy and guard against rapid heat loss. This mechanism is a built-in survival response, reducing metabolic rate and prolonging endurance in cold conditions.

Dual Nervous System Response: Stress Adaptation and Calming From Parasympathetic Activation After Cold Shock

After the initial shock, the co-activation of sympathetic and parasympathetic systems creates both an adaptation-forcing stress and an underlying calming effect, optimizing both vigilance and recovery. This physiological sequence underpins many of the health benefits of cold exposure.

Parasympathetic Timing Explains Divergent Outcomes of Long vs. Brief Cold Immersions

Variations in the duration of cold immersion determine whether the stress or recovery response dominates, explaining the differences in outcomes observed between short and prolonged exposures.

Brown Fat Tissue Is the Main Engine For Heat Generation, With Mitochondria Activated by Noradrenaline

Brown fat generates heat effici ...

Minimum Effective Dose: 11 Minutes Cold and 57 Minutes Heat Exposure Weekly Across Sessions

Susanna Søberg seeks to identify the smallest amount of cold exposure required to deliver measurable health benefits. In her study, she recruits winter swimmers who swam regularly for two to three seasons but only stayed in cold water for one to two minutes per session, two to three times a week. Each visit included three rounds of cold-water dips interspersed with two sauna sessions. This routine totaled 11 minutes of cold exposure per week and around 57 minutes of heat exposure (approximately 10 to 15 minutes per sauna session). These combined exposures prove to be a balanced form of hormetic stress—small, manageable challenges that stimulate beneficial cellular adaptation.

The data show that this moderate approach is effective for cardiovascular health and metabolic benefits. Even with limited time and resources, a person can achieve significant metabolic and cardiovascular improvements by simply alternating short cold exposures with brief heat exposures. This reduces the risk of overtraining and helps avoid excessive physiological stress that could raise cardiovascular disease risk. Deliberate temperature changes just above the minimum effective dose are sufficient—there is no need to expose oneself to extreme conditions or long durations.

Temperature Variation Key for Cellular Adaptation Over Specific Temperatures, Enhancing Protocol Flexibility and Accessibility

Søberg and Huberman agree that the change in temperature—moving from warm to cold or cold to warm—is more important than strict adherence to absolute values. Temperature variation triggers adaptation in cells, stimulates brown fat, and enhances metabolic health. For example, sleeping in a 19°C room even while under a duvet and with clothes on has been shown (Hans Niederl’s 2017 study) to increase [restricted term] sensitivity and brown fat growth. Returning to warmer environments reverses these gains, underscoring the role of consistent, moderate cold exposure rather than mere static low temperatures. Even cold air exposure to the face during sleep can activate brown fat, as the face contains a dense array of cold receptors.

Not only whole-body immersion but even partial exposure, like putting one’s hand in cold water, has been shown to stimulate brown fat activation. The crucial factor is the dynamic shift in skin temperature—people who experience outdoor temperature fluctuations or move frequently between varied climates show more robust brown fat activation than those exposed to constant temperatures.

Huberman and Søberg stress the flexibility and personalization of protocols: it is not necessary to find or maintain specific cold-water or sauna temperatures. Regularly shifting between heat and cold, and not being rigidly prescriptive about exact conditions, offers more benefit and accessibility for most people.

Cold Exposure Should Cause Discomfort and the Cold Shock Response, but Avoid Extreme Conditions or Prolonged Hypothermia

Effective cold exposure is uncomfortable and should trigger the cold shock response—hyperventilation or gasping—for beginners. This sympathetic nerve activation is part of the hormetic stress necessary for adaptation. Over time, with repeated practice, the body and nervous system adapt; the shock response diminishes, and exposure becomes less uncomfortable. This adaptation ...

Exposure to cold and heat brings a suite of benefits for metabolic health, inflammation reduction, cardiovascular function, and even mental clarity and vitality. Susanna Søberg’s research highlights the profound ways that simple environmental stresses—cold plunges, winter swimming, or even thermal adjustments while sleeping—can recalibrate core aspects of human physiology and psychology.

Cold and Heat Exposure Enhance Metabolism, Improving [restricted term] Sensitivity

Studies reveal that both cold and heat exposures significantly improve [restricted term] sensitivity. Søberg references a 2016 study where middle-aged winter swimmers, after one season of regular cold-water immersion, exhibited increased [restricted term] sensitivity. Specifically, these swimmers produced less [restricted term] at rest and in response to a glucose drink. Yet, their bodies cleared glucose from the bloodstream more rapidly than non-swimmers, indicating their cells required less [restricted term] to manage sugar—a hallmark of optimal metabolic health.

This pattern persists beyond acute exposure: fasting winter swimmers, measured after eight hours without food and while resting, maintained lower [restricted term] levels and quicker glucose clearance compared to controls. A similar benefit is observed from mild cold exposure at night; individuals sleeping at 19°C showed increased [restricted term] sensitivity, but this advantage reversed when they returned to customary warmer sleeping temperatures, corresponding with reduced brown fat activation.

Improved [restricted term] sensitivity supports better glucose balance, lowering the risks for type 2 diabetes and other metabolic diseases. By helping the body process sugar effectively with less hormonal intervention, cold- and heat-adapted individuals reduce their likelihood of developing [restricted term] resistance—a key metabolic risk factor.

Cold and Heat Exposure Reduce Inflammation, Lower Blood Pressure, Heart Rate, and Cytokines

Exposure to temperature variation also combats inflammation and improves cardiovascular markers. In winter swimming studies, participants consistently showed lower blood pressure and reduced resting heart rate after the season, both strong indicators of cardiovascular health and reduced systemic stress.

Søberg’s studies tracked inflammatory markers such as interleukin-6 (IL-6), which surged with cold exposure only to be succeeded by anti-inflammatory responses, including elevated IL-10. This sequence produces a net reduction in inflammation, positioning cold and heat exposure as strategies to open "an open door for preventing lifestyle diseases."

The reductions in blood pressure and heart rate documented with regular exposure indicate more efficient cardiovascular function and lower levels of stress hormones. Less inflammation and better vascular function can delay the onset of metabolic disorders and protect cardiovascular health.

Cold and Heat Exposure's Anti-Inflammatory Effects May Protect Against Lifestyle, Metabolic, Mental, and Neurodegenerative Diseases

Chronic inflammation is a pivotal factor in the development of not only metabolic disorders but mental health conditions and neurodegenerative diseases. Søberg refers to emerging research connecting high inflammation to increased risks of depression, anxiety, and Alzheimer’s disease. By harnessing natural temperature stressors, adults can potentially reduce the long-term risk of such conditions.

Modern sedentary lifestyles—marked by climate insulation, reduced movement, and limited environmental variation—have shifted humans out of the cyclical thermal challenges that previously maintained physiological r ...

The Søberg Principle, as described by Andrew Huberman and Susanna Søberg, emphasizes the importance of ending cold exposure sessions in cold water to maximize the body’s thermogenic and metabolic benefits. By forcing the body to independently restore core temperature after finishing in the cold, this principle transforms brief stimuli into prolonged periods of enhanced metabolism and resilience.

Cold Exposure Ending With Water Induces Prolonged Independent Thermogenesis

Ending on cold requires the body to heat up by itself, which stimulates both brown fat and muscles to sustain thermogenesis. Susanna Søberg likens this process to a training session not just for muscles but also for cells and metabolism: shivering and the stress of cold create a healthy challenge—known as hormetic stress—that makes cells more robust and improves their future responses.

This forced recovery after cold ensures that brown fat and muscles remain active for hours after the exposure. Rather than relying on external warming, the body sustains internal metabolic activity, extending the benefits of a one- or two-minute cold plunge into a lasting metabolic exercise. The result is an extended elevation in metabolism, allowing practitioners to burn additional calories and train their thermogenic tissues long after the cold exposure ends.

Extended Thermogenesis Enhances Neurotransmitter Levels and Increases Heat Tissue Efficiency and Mitochondrial Density

The Søberg Principle maintains elevated levels of noradrenaline and similar thermogenic neurotransmitters well after the actual cold session, as the body remains cold for longer and needs continued activation to generate heat. This demand for sustained heat increases not only the efficiency of brown fat and muscle but also boosts mitochondrial density in these cells, making them more effective in generating heat rapidly.

With repeated practice—such as three to five sessions—individuals often notice they become better at staying warm, as their body adapts and maintains a higher core temperature with more ease. This adaptation comes from greater mitochondrial biogenesis and enhanced tissue performance, effects that are much stronger when ending on cold than with passive rewarming methods.

Søberg Principle Boosts Adherence and Effectiveness With Lasting Benefits From Brief Commitments

Following the Søberg Principle, practitioners find lasting metabolic and health benefits, even from short, feasible sessions. By ending in the cold and avoiding the temptation of immediately returning to a hot shower or warm environment, people preserve the critical period where metabolism and cellular activation remain high. This approach supports brief, accessible cold exposure sessions that ...