and Dr Andrew Huberman with Dr Susanna Søberg
Dr. Susanna Søberg, a leading expert on deliberate cold and heat exposure from the University of Copenhagen, highlights that while there's a growing awareness of these practices, modern society's pursuit of comfort has led to a reluctance to embrace beneficial stressors. Her research, including a seminal study published in Cell Reports Medicine in 2021, and her book "Winter Swimming," aim to provide scientific backing for these ancient practices, moving beyond anecdotes.
1. The "Why" We Need Cold and Heat Exposure
- Against Modern Comfort and Stress: Our society is "overwhelmed" by stress, leading to increasing rates of depression and lifestyle diseases. We have engineered discomfort out of our lives, from temperature-controlled environments to instant food delivery, narrowing our "window of tolerance" to stress.
- Evolutionary Imperative: Humans are "evolved with the cold" and our ancestors embraced temperature changes daily. This historical context suggests that exposure to cold and heat is fundamental for bodily and mental "reset" and optimal health.
- Holistic Benefits: Deliberate cold and heat exposure offer benefits across physical health (metabolism, weight, immune system, pain, cardiovascular health), mental well-being (resilience, mood, anxiety, depression, confidence), and athletic performance (recovery, endurance).
2. Physiological Mechanisms and Benefits
- Activation of the Nervous System:
- Sympathetic (Stress) Response: Cold immersion rapidly activates the sympathetic nervous system, increasing heart rate, alertness, and releasing catecholamines like epinephrine, norepinephrine, and dopamine. Noradrenaline levels can increase by up to 250% above baseline within minutes, lasting for hours.
- Dopamine Boost: Dopamine, crucial for drive, motivation, and a good mood, is also released and sustained for up to four hours after cold exposure, offering a natural "high" without the crash associated with substances like cocaine or alcohol.
- Parasympathetic (Calming) Response: Submerging up to the neck in cold water can activate the diving response, which slows heart rate and oxygen consumption, promoting a calming, "rest and digest" state. This unique activation of both sympathetic and parasympathetic systems simultaneously "exercises" and strengthens the nervous system.
- Brown Adipose Tissue (BAT) / Brown Fat:
- Inner Heater: Brown fat is a "healthy", mystifying organ that serves as the body's primary "inner heater" and temperature regulator. Unlike white fat (energy storage), brown fat is rich in mitochondria (energy factories) that burn glucose and fat from the bloodstream to generate heat.
- Metabolic Boost: Activating brown fat increases metabolism, improves insulin sensitivity, and helps clear sugar and fat from the bloodstream, contributing to less obesity and a lower risk of Type 2 Diabetes.
- Growth and Decline: Babies are born with a lot of brown fat. While it naturally decreases with age, studies show that regular cold exposure can increase the amount and efficiency of brown fat even in adults. Conversely, a lack of cold exposure can lead to its decline, correlating with increased obesity and lifestyle diseases after age 40.
- Easy Activation: Brown fat can be activated by even subtle cold stimuli, such as a hand in cold water, being outdoors in a t-shirt, wearing a cooling vest, or simply sleeping in a cool room (e.g., 19°C).
- Shivering and Afterdrop: Shivering is a muscle-driven mechanism to generate heat, burning calories and activating brown fat. The "afterdrop" refers to the body's core temperature continuing to drop after leaving cold water as peripheral blood returns to the core, leading to further shivering. This prolonged heat generation is a beneficial "exercise" for cells, promoting "healthy stress" or hormesis.
- Cardiovascular and Immune Health: Regular cold exposure, often in conjunction with heat, can lead to lower blood pressure and heart rate. Cold showers (even as short as 30 seconds) have been shown to reduce sick days from work, suggesting a boost to the immune system.
3. Mental and Psychological Benefits
- Increased Resilience and Confidence: Deliberately facing uncomfortable cold builds mental resilience and increases one's "stress threshold". It's a "mental workout" that teaches you to push through discomfort, leading to greater confidence in handling other life stressors.
- Improved Mood and Presence: Cold exposure can alleviate symptoms of depression and anxiety. It forces you into the present moment, making it impossible to dwell on worries or distractions, providing a unique "perspective" and sense of mental balance.
4. Protocols and Thresholds (The Søberg Principle)
- Minimum Effective Dose: The goal is to achieve an "uncomfortably cold" "cold shock" to activate the body's adaptive responses, rather than enduring extreme cold for prolonged periods.
- Cold Exposure: Dr. Søberg's research suggests that 11 minutes of deliberate cold exposure per week is sufficient for health benefits, divided into 2-3 sessions of 1-2 minutes per dip.
- For cold showers, 30-90 seconds at the end of a warm shower can significantly reduce sick days.
- Water temperature of 15°C or below is considered cold. Even colder air (e.g., 19°C) can activate brown fat.
- Overdoing it (e.g., 30 minutes in ice water) can exhaust cells, flatten, or even reverse benefits.
- Heat Exposure (Sauna): 57 minutes of sauna per week is recommended when combined with cold, divided into 2-3 sessions of 10-15 minutes each.
- Finnish cohort studies support benefits from 19-30 minutes per session, noting that longer durations (over 30 minutes) do not provide additional health benefits and may even increase risks.
- Contrast Therapy: Alternating between cold and heat (e.g., cold dip then sauna) is highly effective, strengthening cells and improving blood circulation.
- The Søberg Principle: End on Cold: A crucial aspect of the protocol is to always end a session on cold. This forces the body to generate its own heat, keeping brown fat and muscles activated for hours, thereby increasing metabolism and sustaining neurotransmitter benefits.
5. Individual Differences and Safety
- Varying Tolerance: Cold tolerance varies greatly among individuals due to nervous system sensitivity, body composition, age, and ethnicity. It's not a competition, and comparing oneself to others is counterproductive and potentially dangerous.
- Gender Differences: Women typically have more brown fat but also run colder peripherally (hands, feet, ears) and have a higher preferred comfortable temperature (24°C vs. 22°C for men).
- Children: Children are smaller and have a higher surface area to mass ratio, making them more susceptible to hypothermia. Brief dips (e.g., a quick dip) are advised, and children should always be supervised by an adult, learning to listen to their own body's signals.
- Head Immersion: While some dunk their heads, it's not strictly necessary for benefits and can increase heat loss from the core by 36% and decrease blood flow to the brain, potentially causing dizziness. Wearing a beanie can mitigate this.
- Practical Adaptations: If hands or feet are painful, keeping them out of the water or wearing neoprene gloves/booties is acceptable, as the body will still receive significant cold exposure.
- Risk of Hypothermia: Cold is a powerful stimulus, and prolonged exposure can lead to hypothermia, which is a serious risk. It's vital to "listen to your own body" and know when to exit the cold.
In essence, Dr. Søberg advocates for a deliberate, controlled, and consistent approach to cold and heat exposure, emphasizing that even small, regular doses of "healthy stress" can unlock profound physiological and psychological benefits, helping to "rewire" our bodies and minds for greater health and resilience.