Cold Plunge Benefits: The Søberg Study, the Evidence and Safety

A cold plunge is a short, deliberate immersion in cold water, usually around 15 °C (59 °F) or colder, in a plunge tub, an ice bath, a lake or the sea, typically for one to a few minutes. The cold plunge benefits that research supports so far are modest and short-term: a large surge in stress hormones that people experience as alertness, and lower self-reported stress for some hours afterwards. Claims about lasting fat loss, stronger immunity or any effect on disease are not supported by the current evidence.

It matters because cold plunging has gone from a niche Nordic habit to a mainstream wellness trend in a few years, carried by podcasts, social media and one small Danish study that is quoted far more confidently than its authors ever intended. Cold water is also a genuine physical hazard. Knowing what the studies measured, and what the first minute in cold water does to breathing and heart rhythm, matters before anyone climbs in.

Safety first. The first seconds in cold water trigger cold shock: an involuntary gasp, rapid uncontrolled breathing and a sharp rise in heart rate and blood pressure. Enter gradually, keep immersions short, never plunge alone or after drinking alcohol, and never combine cold water with breath holds or fast "power breathing". Do not judge how long to stay in by how cold you feel. Anyone with a heart condition, high blood pressure, a heart-rhythm disorder (including long QT syndrome), Raynaud's phenomenon, who is pregnant or who takes regular medication should consult a clinician before trying cold-water immersion.

What is a cold plunge?

A cold plunge is any brief, intentional immersion of most of the body in cold water, and the umbrella term researchers use is cold-water immersion, or deliberate cold exposure when cold showers are included. People do it in purpose-built tubs, in baths filled with ice, in open water through the winter, or simply by finishing a shower on the cold setting.

The practice is old. Winter swimming and ice-hole bathing have long traditions in the Nordic countries and Russia, often paired with the sauna, and athletes have used ice baths for decades as a recovery tool. The recent boom draws on three sources: the Dutch athlete Wim Hof, whose method combines breathing, cold and mindset (covered in detail in the breathwork and Wim Hof method guide); the Huberman Lab podcast, which popularised cold exposure as a daily protocol; and the Danish researcher Susanna Søberg, whose study of winter swimmers is the source of the most widely repeated number in the field.

The UK's Royal National Lifeboat Institution treats water below 15 °C as cold, and many plunge enthusiasts go well below 10 °C.

What did the Søberg study actually find?

The Søberg study found that a small group of young male winter swimmers regulated their body temperature differently from matched non-swimmers. It was published as Søberg and colleagues, "Altered brown fat thermoregulation and enhanced cold-induced thermogenesis in young, healthy, winter-swimming men", in Cell Reports Medicine in October 2021.

The researchers compared eight young, healthy men who swam in cold water through the winter with eight matched men who did not. The swimmers typically combined cold dips with sauna sessions, and across a week they reported roughly 11 minutes in cold water and about 57 minutes in the sauna, spread over a few sessions.

Three findings stand out:

• Lower comfortable core temperature. At a temperature they found comfortable, the winter swimmers had a lower core body temperature than the controls.
• Quieter brown fat at comfortable temperatures. Brown adipose tissue, the heat-producing fat that the body recruits in cold conditions, showed no measurable activity in the swimmers at thermal comfort.
• More heat production during cooling. When the men were cooled, the swimmers produced more heat (cold-induced thermogenesis) than the controls.

The limits matter as much as the results. The sample was very small, included only men, and consisted of people who had chosen to winter-swim and might differ from non-swimmers in diet, fitness, genetics or other habits. Because the study observed existing swimmers rather than assigning people to a routine, it cannot show that winter swimming caused the differences. The popular "11 minutes a week" target comes from this description of what the swimmers happened to do. It was never tested as a dose, and no study has shown it to be an optimal or minimum amount.

What happens in the body during a cold plunge?

Cold water triggers a fast, strong stress response, driven by the sympathetic nervous system and the release of catecholamines such as noradrenaline (also called norepinephrine) and dopamine. Heart rate and blood pressure rise, blood vessels in the skin constrict to conserve heat, breathing quickens, and the body starts producing extra heat through shivering and other mechanisms.

The figures most often quoted come from Šrámek and colleagues, "Human physiological responses to immersion into water of different temperatures", European Journal of Applied Physiology (2000). Young men sat in water up to the neck for one hour. At 14 °C, plasma noradrenaline rose by about 530 percent, dopamine by about 250 percent and metabolic rate by about 350 percent. At 32 °C, metabolic rate did not change.

Those numbers need context. They describe a full hour of head-out immersion at 14 °C, far longer than a typical one- to three-minute plunge, so they should not be quoted as the effect of a short dip. What the study does support is the general direction: cold water reliably produces a large catecholamine response, which is consistent with the alert, energised feeling many people describe afterwards.

What does the evidence support so far?

The best summary of the human evidence is cautious: some short-term changes in how people feel, a temporary rise in inflammatory markers, and very little support for broader claims.

Cain and colleagues at the University of South Australia published "Effects of cold-water immersion on health and wellbeing: A systematic review and meta-analysis" in PLOS ONE in January 2025. They pooled 11 randomised studies with 3,177 healthy adults. Their main findings:

• Stress. Self-reported stress was lower 12 hours after immersion, but not immediately, at one hour, or at 24 or 48 hours.
• Sleep. One study of 20 men reported better sleep quality after cold-water immersion than after passive recovery; the data were male-only and came from a training context.
• Quality of life. In a large cold-shower trial, a small improvement in a mental-wellbeing score at 30 days was no longer significant at about three months.
• Inflammation. Inflammatory markers rose immediately and an hour after immersion, consistent with an acute stress response.
• Immunity and mood. The evidence was thin: no significant immune changes in the pooled data and no measurable mood difference in the one study that assessed it.

The authors judged the evidence to be limited by small samples, mostly male participants and mostly single immersions, and they called for larger, higher-quality trials of long-term effects. For a broader look at how to weigh studies like these, see biohacking for beginners: what the evidence supports.

What cold plunges do not do

The current evidence does not show that cold plunges produce lasting fat or weight loss, "boost" the immune system, or treat or prevent any disease. Brown fat activity and heat production rise during cold exposure, but that does not translate into demonstrated long-term changes in body weight in controlled human trials.

There is also a trade-off for people who lift weights. Roberts and colleagues, writing in the Journal of Physiology (2015), had men complete 12 weeks of strength training followed either by 10 minutes of cold-water immersion or by active recovery on a stationary bike. The cold-water group gained less muscle mass and strength. For that reason, people training for muscle often keep cold plunges away from their lifting sessions, for example on rest days or several hours apart.

How do you stay safe in cold water?

Staying safe means respecting the first minute in cold water, keeping immersions short and never plunging alone. The main risks are well documented by Professor Mike Tipton and colleagues at the University of Portsmouth's Extreme Environments Laboratory.

Cold shock. Sudden immersion triggers an involuntary gasp, rapid uncontrolled breathing and a surge in heart rate and blood pressure. The response is strongest in the first minute or so, and Tipton's work shows it peaks in water between about 10 and 15 °C, temperatures many people assume are mild. A gasp with the face under water can mean inhaling water.

Autonomic conflict. Cold on the skin drives the heart faster, while cold water on the face and breath holding trigger the diving response, which slows it. Shattock and Tipton (Journal of Physiology, 2012) described how activating both at once can disturb heart rhythm, a particular concern for people with underlying heart conditions such as long QT syndrome. Keeping the face out of the water and never holding the breath during a plunge reduces this conflict.

Continued cooling. Core temperature can keep falling after you get out, a phenomenon called afterdrop, so warming up gradually with dry clothes and movement matters more than a hot shower straight away.

Practical rules that follow from this research:

• Enter slowly and let the gasp pass before you settle, keeping your head and face above water.
• Keep immersions short, and set a timer instead of judging time by how cold you feel; numbness and confusion arrive before you notice them.
• Never plunge alone, and never after alcohol.
• Never combine cold water with breath holds or hyperventilation.
• Get out if you feel chest pain, dizziness or an irregular heartbeat.

This article does not recommend any tub, chiller or brand.

Billy Carson's reading

4biddenknowledge has not published a Billy Carson video, post or podcast episode dedicated to cold plunges or ice baths, so this article does not attribute any view, routine or claim about cold exposure to him. Instead, it sits within the site's broader biohacking coverage, which treats practices like this one by setting out what proponents say alongside what the studies measured.

Readers who want the closest related material on the site can start with the breathwork and Wim Hof guide, which covers the controlled cold-training studies, and with April Carson's earlier overview, The Rise of Biohacking: Optimizing Your Health with Technology. Because so much of the cold-plunge conversation is really about how people handle stress, Barry Nembhard's article What Are You Doing With Stress? on healthy and unhealthy coping strategies is a useful companion read.

Watch, read and go deeper

The biohacking topic hub collects every evidence-led guide on the site in one place. From there, the breathwork and Wim Hof method guide is the natural next step, since cold and breathing are usually practised together and the safety rules overlap. For other practices that follow the same claims-versus-evidence format, read grounding and earthing explained and red light therapy explained. Newcomers should start with biohacking for beginners.

Frequently asked questions

What are the proven cold plunge benefits?

The best-supported effects are short-term. Cold water produces a large stress-hormone response linked to feeling alert, and a 2025 meta-analysis found lower self-reported stress 12 hours after immersion. Evidence for better sleep and quality of life is limited, and there is very little support for immune or mood claims.

Is 11 minutes a week the right amount of cold exposure?

No study has tested 11 minutes a week as a dose. The figure describes what a small group of Danish winter swimmers reported doing in Susanna Søberg's 2021 observational study, alongside about 57 minutes of sauna. It is a description of their habits, not a proven target.

How cold should a cold plunge be?

Water below about 15 °C is generally treated as cold, and many people plunge colder. Colder water does not mean more benefit; it means a stronger cold shock and faster heat loss. Cold shock peaks in water between about 10 and 15 °C, so even moderate temperatures deserve caution.

Do cold plunges burn fat?

Cold exposure activates brown fat and raises heat production while you are cold, but controlled human trials have not shown that cold plunges lead to lasting fat or weight loss. Claims that they "burn fat" go well beyond the evidence.

Should I take a cold plunge after lifting weights?

People training for muscle size and strength often avoid it. A 2015 study in the Journal of Physiology found that 10 minutes of cold-water immersion after each strength session over 12 weeks led to smaller gains in muscle mass and strength than active recovery. Separating plunges from lifting sessions is a common compromise.

Sources and further reading

• Søberg S. et al., "Altered brown fat thermoregulation and enhanced cold-induced thermogenesis in young, healthy, winter-swimming men," Cell Reports Medicine, October 2021.
• Šrámek P. et al., "Human physiological responses to immersion into water of different temperatures," European Journal of Applied Physiology 81, 2000.
• Cain T. et al., "Effects of cold-water immersion on health and wellbeing: A systematic review and meta-analysis," PLOS ONE, January 2025.
• Roberts L.A. et al., "Post-exercise cold water immersion attenuates acute anabolic signalling and long-term adaptations in muscle to strength training," Journal of Physiology, 2015.
• Shattock M.J. and Tipton M.J., "'Autonomic conflict': a different way to die during cold water immersion?" Journal of Physiology, 2012.
• Tipton M.J. et al., "Cold water immersion: kill or cure?" Experimental Physiology, 2017.

Anyone considering cold-water immersion, particularly with a heart, blood-pressure or heart-rhythm condition, Raynaud's phenomenon, a pregnancy or regular medication, should consult a clinician first.

These statements have not been evaluated by the Food and Drug Administration. This content is for information only and is not medical advice.