How to Use Cold & Heat Exposure to Improve Your Health
Summary
Dr. Susanna Søberg, who completed her doctoral research at the University of Copenhagen, discusses the science behind deliberate cold and heat exposure and their effects on metabolism, brown fat activation, neurotransmitter production, and long-term health outcomes. Her landmark 2021 study in Cell Reports Medicine established minimum effective thresholds for cold and heat exposure to drive meaningful physiological changes. The conversation covers everything from the cold shock response to specific protocols for combining sauna and cold immersion.
Key Takeaways
- Any uncomfortable cold exposure — cold plunge, cold shower, cold air, or even cold wind — is sufficient to activate brown adipose tissue and trigger beneficial metabolic responses.
- Shivering is beneficial, not something to avoid; it activates brown fat, burns calories, and improves insulin sensitivity through healthy cellular stress (hormesis).
- End cold-heat sessions on cold (the “Søberg Principle”) to maximize metabolic benefits by forcing the body to self-reheat using brown fat thermogenesis.
- The cold shock response — hyperventilation, gasping, elevated heart rate — subsides with repeated exposure as the body adapts; this adaptation is a feature, not a bug.
- Sleeping in a room at 19°C for one month has been shown to increase brown fat volume and improve insulin sensitivity, with no need for immersion.
- Long-term cold exposure is associated with lower blood pressure, lower resting heart rate, and improved insulin sensitivity — markers that reduce risk of cardiovascular and metabolic disease.
- Brown fat is highly plastic: it can grow with repeated cold exposure and shrink with prolonged warmth; it remains activatable into older age, especially in people who work or move outdoors.
- Women have more brown fat than men and are thermally comfortable at approximately 2°C warmer room temperatures, which has implications for study design and protocol personalization.
- The minimum effective protocol studied was 2–3 sessions per week, with cold immersion of 1–2 minutes per dip, three rounds of cold per session, alternated with ~10–15 minutes of sauna at ~80°C.
Detailed Notes
What Happens Physiologically When You Enter Cold Water
When the body is submerged in cold water, cold receptors in the skin rapidly signal the hypothalamus, triggering:
- Activation of the sympathetic nervous system
- A sharp rise in catecholamines: norepinephrine, epinephrine, and dopamine
- Cold shock response: hyperventilation, gasping reflex, elevated heart rate and blood pressure
- Vasoconstriction in peripheral blood vessels to protect core temperature
If you are already warm (e.g., coming from a sauna), the shock is perceived as less intense because the thermal differential at the skin is reduced. With repeated exposure, the shock response diminishes as the body adapts — the hyperventilation subsides, cold becomes more tolerable, and resilience builds.
Cold Modalities: Comparing Effectiveness
| Modality | Cold Receptor Activation | Brown Fat Activation | Notes |
|---|---|---|---|
| Cold immersion (to neck) | Very high | Strong | Most studied; most potent |
| Cold shower | Moderate | Likely effective | Fewer studies; harder to control |
| Cold air / t-shirt in cold wind | Lower | Present | Activates sympathetic NS; practical for daily use |
| Submerging face in ice water | High (face receptors) | Present | Activates dive reflex |
| Sleeping at 19°C | Low but sustained | Documented increase | One month sufficient to grow brown fat |
Key point: Cold immersion covers more body surface area with molecules closer to skin, producing a more rapid and potent autonomic response than cold air. However, even cold air activates norepinephrine release and brown fat thermogenesis.
Brown Fat: Biology and Function
Brown adipose tissue (BAT) differs from white fat in its high mitochondria density, which gives it its color and enables heat production (non-shivering thermogenesis).
Location in the body:
- Primarily in supraclavicular region (under collarbone), upper back, and around the spine
- Close to the skin surface — measurable via infrared thermography
- At least six depots identified in humans; distribution is more widespread than previously thought
Key properties:
- Highly plastic: can grow (hyperplasia of mitochondria, increased cell volume) and regress
- Activated primarily by cold and norepinephrine
- Acts as a “first responder” to temperature change — faster than skeletal muscle shivering
- Takes up glucose and free fatty acids from the bloodstream to fuel thermogenesis, directly improving insulin sensitivity and blood glucose regulation
Three parallel activation pathways:
- Cold receptors in skin → hypothalamus → norepinephrine release → brown fat activation
- Direct pathway: cold skin receptors → brown fat (bypassing central processing)
- Shivering in skeletal muscle → additional brown fat activation via succinate signaling
Association with aging and obesity:
- Brown fat activity tends to decrease after age 40
- Associated with increased insulin resistance and obesity — though causality is unclear (does less BAT cause obesity, or does obesity reduce BAT?)
- People who work outdoors physically tend to maintain more brown fat into older age
The Role of Shivering
Shivering thermogenesis is a secondary metabolic response to cold:
- Occurs as the after drop — when core body temperature continues to fall after exiting cold water because vasoconstriction releases and cool peripheral blood returns to the core
- Shivering increases metabolism, burns calories, and activates brown fat
- Produces hormetic stress in muscle cells, increasing heat shock proteins and cellular robustness
- Improving insulin sensitivity is one documented outcome of repeated shiver-inducing cold exposure
Practical note: Do not rush to warm up after cold exposure. The shiver response is part of the beneficial mechanism.
The Søberg Study: Design and Protocol
Population: Male winter swimmers, already adapted (2–3 prior seasons), matched with sedentary controls on BMI and diet.
Protocol per session:
- 3 rounds of cold water immersion (1–2 minutes each)
- 2 sauna sessions (~10–15 minutes at ~80°C)
- Alternating cold → sauna → cold → sauna → cold
- Always ending on cold
Frequency: 2–3 sessions per week
Water temperature: Outdoor seawater, seasonally cold (exact temperatures varied)
Primary outcome measured: Brown fat activation via PET-CT scanning
Key finding: This protocol — representing a minimum effective dose — was sufficient to increase brown fat activity, improve glucose metabolism, and produce measurable metabolic benefits.
The Søberg Principle: End on Cold
Ending a session on cold (rather than warming up in sauna or hot shower afterward) forces the body to generate its own heat using brown fat thermogenesis. This is the mechanism through which the metabolic benefit is consolidated.
- Getting into a hot shower immediately after cold immersion short-circuits this process
- The body’s autonomous rewarming is itself the training stimulus for the brown fat
Long-Term Health Benefits of Deliberate Cold Exposure
Based on referenced studies and Dr. Søberg’s own research:
- Lower resting blood pressure
- Lower resting heart rate
- Improved insulin sensitivity and glucose regulation
- Reduced inflammation markers
- Increased catecholamine levels (dopamine, norepinephrine) — effects lasting many hours after exposure
- Improved thermal regulation — greater comfort in cold environments without additional clothing
- Mental health: increased serotonin, better mood, increased energy and positive affect post-exposure (questionnaire-based; mechanistic studies needed)
Cold Adaptation and Practical Entry Points
For those new to cold exposure, a progression might look like:
- Sleep in a cooler room (~19°C) — easy entry, documented benefit
- Wear fewer layers outdoors in autumn/winter
- Cold showers — less studied but likely effective for brown fat activation
- Cold immersion (plunge, open water) — most potent stimulus
You do not need extreme durations. One to two minutes of uncomfortable cold is