Cold Plunge & Cold Water Immersion

Disclaimer: Cited research, not medical advice. Prestige Hyperbaric is a wellness center, not a medical facility. Always consult a qualified healthcare provider before starting any therapy.

Introduction

Cold water immersion (CWI) — practiced under names ranging from ice bath to cold plunge to winter swimming — is among the oldest deliberately applied physical stressors in human health culture. What was once the province of competitive athletes seeking faster muscle recovery has, in the last decade, migrated decisively into the mainstream wellness world. The migration has been driven by a convergence of popular science communication, social media, and a growing body of peer-reviewed research documenting mechanisms that extend far beyond simple soreness relief. Today, a single immersion session can be framed with physiological precision: a hormetic stressor that activates catecholamine cascades, recruits brown adipose thermogenesis, modulates inflammatory signaling, and reshapes autonomic tone — all within minutes.

This chapter reviews the history of cold water therapy, its established and proposed mechanisms of action, the evidence base supporting specific wellness outcomes, practical protocols derived from that evidence, the critical caveat regarding timing around resistance training, and the safety framework every practitioner should understand before introducing clients to the modality.

Historical Context

The therapeutic use of cold water is ancient. Hippocrates (c. 460–370 BCE) dedicated substantial attention to water temperature in his work De aere, aquis et locis, describing how cold and hot baths differentially affected fever, fatigue, and bodily humor 1. Greek athletes incorporated cold-water recovery baths as part of training at gymnasium complexes, an early recognition that water temperature modulated recovery 2.

The tradition continued through the Roman baths and into medieval monastic communities. The modern era of structured hydrotherapy, however, traces its origin to two nineteenth-century European figures. Vincenz Priessnitz (1799–1851), an Austrian peasant farmer from Gräfenberg (present-day Lázně Jeseník, Czechia), is widely credited as the founder of modern hydrotherapy. With no formal medical training, he nonetheless developed a systematic cold-water treatment protocol — compresses, baths, douches, fresh air, and exercise — that drew patients from across Europe. In 1837 an Imperial Commission certified his practice as "a new remarkable phenomenon in the field of health care," and his clinic treated over 1,500 patients per year at its peak 2.

Sebastian Kneipp (1821–1897), a Bavarian priest, extended Priessnitz's work into a more holistic five-pillar system integrating hydrotherapy, herbal medicine, physical movement, nutrition, and what he called "a proper lifestyle." Kneipp's alternating hot-cold water applications — still used in European spa medicine — were directed at improving circulation, hardening the nervous system, and treating chronic disease. His approach directly influenced the naturopathic tradition and modern balneotherapy 1.

The twentieth century saw cold-water therapy institutionalized in Scandinavian sauna culture (alternating sauna heat with cold-water immersion) and in sports medicine, where ice baths became standard equipment in professional team facilities by the 1990s. The early twenty-first century brought two paradigm shifts. First, the Wim Hof Method — developed by Dutch athlete Wim Hof and formally studied from 2012 onward — demonstrated that voluntary cold exposure combined with structured breathing could influence the autonomic nervous system and even the innate immune response, findings published in Proceedings of the National Academy of Sciences in 2014 3. Second, Andrew Huberman, a Stanford neuroscientist and popular science communicator, synthesized catecholamine, thermoregulatory, and metabolic research into widely circulated protocols, bringing cold plunge into everyday wellness conversation and popularizing the concept of an 11-minute minimum effective weekly dose 4.

Mechanisms of Action

Understanding why cold water immersion produces the effects it does requires tracing the body's response across several interacting physiological systems. These are not isolated events; they cascade and amplify one another, which is why even brief immersion can produce effects that last hours.

Cold Shock Response and the Catecholamine Surge

The first seconds of cold water contact trigger the cold shock response: an involuntary gasp reflex, hyperventilation, tachycardia, and a profound activation of the sympathetic nervous system. Skin thermoreceptors — present at densities far exceeding those of heat receptors — relay rapid signals to the hypothalamus and brainstem, initiating a sympathoadrenal cascade 5.

The primary neurochemical consequence is a dramatic release of catecholamines — norepinephrine (noradrenaline), epinephrine (adrenaline), and dopamine. Šrámek et al. (2000), in a landmark study of human physiological responses to water immersion at different temperatures, found that immersion at 14°C (57°F) increased plasma noradrenaline concentrations by 530% and dopamine by 250% over baseline 6. Epinephrine increases were comparatively modest. At 20°C the metabolic rate doubled and noradrenaline rose substantially, while at 32°C the effect was minimal — demonstrating the critical role of water temperature in driving the neuroendocrine response 6.

These catecholamine elevations are not transient spikes. Subsequent research using shorter sessions — 10 minutes at 14°C — confirmed that norepinephrine, epinephrine, and cortisol rose significantly and remained elevated for several hours after immersion 7. Notably, plasma cortisol tended to decrease in longer one-hour immersions at all temperatures tested in the Šrámek protocol, suggesting that the cortisol response is duration- and temperature-dependent 6.

The dopamine component is of particular significance. Unlike the rapid-spike-and-crash profile produced by many pharmacological dopaminergic stimuli, cold-exposure-induced dopamine appears to rise more gradually and remain elevated for substantially longer periods — a profile that may underlie subjective reports of sustained mental clarity, motivation, and mood elevation 8. Huberman has characterized this as a "long-lasting increase" in dopamine associated with improved focus and cognitive performance, citing immersions at approximately 60°F (15°C) sustained for 60 minutes as producing especially pronounced and prolonged elevations 4.

Vasoconstriction, Reactive Vasodilation, and Lymphatic Pumping

Peripheral vasoconstriction is the body's immediate thermal defense: cutaneous blood flow plummets as blood is shunted centrally to protect core organs. This response increases total peripheral resistance, transiently elevates blood pressure, and reduces tissue temperature at the skin and in superficial muscle compartments 5.

After exiting the cold water, the body enters a phase of reactive vasodilation — sometimes called the "hunting response" — as the sympathetic vasoconstriction releases and peripheral vessels dilate to restore flow. This cyclical constriction-dilation acts as a mechanical pump for interstitial fluid, a key mechanism behind CWI's effectiveness in reducing post-exercise edema and inflammatory byproducts in muscle tissue.

The lymphatic system benefits by a related mechanism. The lymphatic vasculature lacks its own peristaltic pump; it depends on external compression, muscle contraction, and pressure gradients. Cold water causes lymphatic vessels to contract, propelling lymph fluid centrally. A study measuring lymph flow at the ankle demonstrated that application of 1°C water significantly increased lymph evacuation compared with warmer temperatures, and the effect was amplified when mild external compression was added — the principle behind the therapeutic combination of cold and compression wraps 9. Hydrostatic pressure from full-body immersion adds a further consistent compressive force that complements this lymphatic action.

Brown Adipose Tissue Activation, UCP1, and Thermogenesis

Brown adipose tissue (BAT) is a specialized thermogenic organ distinct from white fat. Rather than storing energy, BAT burns it via uncoupled oxidative phosphorylation — a process mediated by uncoupling protein 1 (UCP1), a mitochondrial inner membrane protein that dissipates the proton gradient as heat rather than converting it to ATP 10. BAT activity is stimulated by cold through the sympathetic nervous system: norepinephrine released at sympathetic nerve terminals in BAT activates β3-adrenergic receptors, triggering cAMP-PKA and p38 MAPK signaling cascades that increase UCP1 gene transcription and activate existing UCP1 protein 10.

In adults, active BAT depots are located primarily in the supraclavicular, cervical, and paravertebral regions. Van der Lans et al. (2013) demonstrated that a 10-day cold acclimation protocol in healthy humans significantly increased BAT activity in parallel with increased non-shivering thermogenesis (NST), and that BAT volume grew, indicating recruitment of new brown adipocytes or activation of beige/brite cells in white adipose depots 11. The increase in NST correlated with BAT activity, directly linking cold exposure frequency to thermogenic capacity.

Søberg et al. (2021) studied young men who regularly practiced Scandinavian winter swimming — brief cold-water dips combined with sauna sessions, 2–3 times per week. Rather than simply having more BAT than controls, the winter swimmers showed altered thermoregulatory physiology: a lower resting core temperature, an absence of BAT activity at comfortable ambient temperatures (suggesting the system had been recalibrated), and markedly greater cold-induced thermogenesis when challenged with cold — burning more calories during cooling than non-swimmers. A distinct circadian peak in supraclavicular skin temperature (a BAT activity marker) was observed in winter swimmers between 4:30–5:30 a.m., absent in controls, suggesting that habitual cold exposure reshapes BAT's circadian contribution to thermoregulation 12. The authors proposed winter swimming as "a potential strategy for increasing energy expenditure," a finding with plausible implications for metabolic health and weight management 12.

BAT-associated cold-induced thermogenesis has been estimated at 120–370 kcal/day, representing 15–25% of resting energy expenditure under warm conditions in BAT-active individuals 13. Chronic cold adaptation increases both the number of brown adipocytes and the induction of beige adipocytes within white fat depots, amplifying total thermogenic capacity 13.

Cold Shock Proteins: RBM3 and CIRP

At the cellular level, cold exposure induces expression of cold shock proteins — a conserved family of RNA-binding proteins whose expression is upregulated by mild-to-moderate hypothermia. The two best characterized human cold shock proteins are RNA-binding motif protein 3 (RBM3) and cold-inducible RNA-binding protein (CIRP) 14.

Both proteins are transcriptionally upregulated when cells experience a drop in temperature into the mild hypothermic range (28–34°C), far below the dramatic cooling of an ice bath but potentially achievable in superficial muscle tissue during prolonged cold water exposure. They function primarily by binding and stabilizing mRNAs, promoting their translation at reduced temperatures, thereby maintaining cellular protein synthesis when the normal ribosomal machinery slows 14.

RBM3 is notable for its neuroprotective properties. Animal models have shown that RBM3 induction during therapeutic hypothermia preserves synaptic density and delays neurodegeneration — findings with potential implications for cognitive resilience, though human translation remains preliminary 15. RBM3 expression is exquisitely temperature-sensitive: even a 1°C drop from 37°C to 36°C is sufficient to initiate induction in neural cells 14. CIRP responds faster upon initial cooling (rising within 3 hours, peaking at 12 hours) and has been linked to regulation of circadian rhythm mechanisms, DNA damage responses, and anti-inflammatory effects including potential reduction in NLRP3 inflammasome activity [14, 15].

Vagal Tone, HRV, and Parasympathetic Rebound

Cold water immersion creates a dynamic interplay between the sympathetic and parasympathetic nervous systems that evolves over the duration of a session and in the period afterward. The initial cold shock response is dominated by sympathetic activation — tachycardia, hypertension, hyperventilation. Within 3–5 minutes in most individuals, the acute sympathetic surge diminishes as thermoreceptors adapt and central processing dampens the initial alarm signal 8.

During and after immersion, cold stimulation of facial and body skin also activates the diving reflex — a vagally mediated response that tends to slow the heart rate. The interaction between simultaneously activated sympathetic and parasympathetic inputs creates what researchers have termed "autonomic conflict," which in susceptible individuals can contribute to arrhythmias (discussed further in Safety). In healthy individuals, however, the competing inputs resolve into enhanced autonomic regulation over time.

Post-immersion, several studies document parasympathetic rebound — a measurable improvement in cardiac vagal tone. De Oliveira Ottone et al. found that 15 minutes in 15°C water accelerated post-exercise parasympathetic reactivation compared to warm water 16. A study of handball players in a training camp found that cold-water immersion at 6°C produced significantly higher parasympathetic HRV indices (lnRMSSD, pNN50) during recovery than passive rest 16. This enhanced HRV — a metric associated with improved stress resilience, recovery capacity, and reduced cardiovascular risk — may partly explain why habitual cold exposure practitioners report subjective improvements in stress tolerance and calm.

Hormetic Stress Response and the Nrf2 Pathway

Cold water immersion is a classical example of hormesis: a biphasic dose-response relationship in which a stressor that is harmful in large doses produces beneficial adaptive responses at low-to-moderate doses. Cold exposure generates transient reactive oxygen species (ROS) and activates cellular stress response pathways, among which the Nrf2-Keap1 pathway is central.

Under baseline conditions, the transcription factor Nrf2 is sequestered in the cytoplasm by Keap1 and rapidly degraded. When oxidative stress or electrophilic stress disrupts Keap1's cysteine residues, Nrf2 translocates to the nucleus and binds antioxidant response elements (ARE) in the promoters of cytoprotective genes — including superoxide dismutase (SOD-1, SOD-2), heme oxygenase-1, glutathione S-transferases, and NAD(P)H quinone oxidoreductase 17. The net effect is an upregulation of the cell's endogenous antioxidant and detoxification capacity.

Animal studies demonstrate that initial cold exposure activates Nrf2 and increases antioxidant enzyme expression; prolonged or extreme cold eventually suppresses Nrf2 and increases apoptotic signaling — underscoring the importance of dose and duration in the hormetic equation 17. At the wellness doses used by healthy individuals (10–15 minutes in 10–15°C water), the stress is designed to be acute and beneficial rather than chronic and damaging.

Inflammation Modulation: NLRP3, Cytokines, and Resolution

The relationship between CWI and inflammation is nuanced and biphasic, which has generated considerable scientific discussion. Immediately after and one hour post-immersion, meta-analyses detect a significant increase in inflammatory markers (SMD: 1.03 at 0 hours; 1.26 at 1 hour post-CWI) — consistent with CWI acting as a genuine physical stressor that initially amplifies rather than suppresses the acute inflammatory response 18.

However, beyond the acute phase, the picture shifts. At 12 hours post-immersion, stress markers (cortisol, perceived stress) are significantly reduced 18. Kox et al.'s 2014 Wim Hof Method study demonstrated that trained practitioners, using a combination of cold exposure and breathing techniques, had proinflammatory cytokines (TNF-α, IL-6, IL-8) that were roughly 50% lower following experimental endotoxemia challenge, with anti-inflammatory IL-10 approximately 200% higher — demonstrating that habituated cold exposure reshapes the immune response to inflammatory challenge 3.

The NLRP3 inflammasome — a multiprotein complex that drives the processing and release of IL-1β and IL-18, and is implicated in chronic inflammatory diseases — may be one target of cold-mediated immune modulation. NLRP3 is highly sensitive to temperature, and emerging research suggests that temperature fluctuations influence its assembly and activity 19. CIRP, the cold shock protein, also has direct regulatory interactions with NLRP3 signaling, though the direction of effect depends on cell type and context 14.

For athletes, the practical implication is that CWI's anti-inflammatory action during recovery — reducing local prostaglandin signaling, tissue edema, and pain receptor stimulation — comes at the cost of blunting some of the inflammatory signals that are necessary for full anabolic adaptation (addressed in detail in the Training Considerations section).

Benefits and Evidence

Recovery and Reduction of Delayed Onset Muscle Soreness (DOMS)

The most extensively studied wellness application of CWI is accelerating recovery from exercise-induced muscle damage (EIMD). A 2025 network meta-analysis evaluating different cold water immersion doses found that medium-duration, moderate-temperature CWI (10–15 minutes at 11–15°C) was the most effective protocol for reducing DOMS, while medium-duration, lower-temperature CWI (10–15 minutes at 5–10°C) best reduced creatine kinase (CK, a biomarker of muscle damage) and improved neuromuscular recovery (jump performance) 20. Both approaches significantly outperformed control conditions. An earlier meta-analysis comparing CWI with whole-body cryotherapy found CWI more effective for short-term DOMS relief within 24 hours (mean difference = 1.07, 95% CI: 0.70–1.43, p < 0.00001) 21.

Mechanisms of recovery benefit include: vasoconstriction reducing local blood flow and accumulation of inflammatory mediators; decreased tissue temperature slowing local metabolism and reducing edema; hydrostatic pressure facilitating lymphatic clearance of cellular debris; and the neural effects of cold reducing pain receptor sensitivity 20.

The Hypertrophy Caveat

For individuals whose primary training goal is muscle hypertrophy or maximum strength development, the recovery benefits of CWI come with a significant trade-off — one that is well-established in the literature and must inform client protocol guidance.

Roberts et al. (2015), in a seminal two-part study published in The Journal of Physiology, found that 12 weeks of strength training with post-session CWI (10 minutes at 10°C) resulted in significantly smaller gains in muscle mass and strength compared to active recovery [22]:

• Muscle mass increase: 309 ± 73 g (active recovery) vs. 103 ± 71 g (CWI group), p < 0.001
• Type II fibre cross-sectional area increased 17% in active recovery; failed to increase significantly in CWI
• Myonuclei per fibre increased 26% in active recovery; no significant increase in CWI
• 1RM leg press strength: 201 ± 65 kg (active recovery) vs. 133 ± 43 kg (CWI)

Molecular analysis revealed that CWI blunted activation of satellite cells and reduced phosphorylation of key mTOR pathway proteins (p70S6K, 4E-BP1) in the 24–48 hours following exercise — the precise signaling window during which hypertrophic adaptation is established 22. The authors concluded that "the use of CWI as a regular post-exercise recovery strategy should be reconsidered" for strength athletes 22.

A 2019 replication by Fyfe et al. confirmed that CWI attenuated muscle fibre hypertrophy (type II fibre CSA reduced by ~1,959 μm² compared to active recovery), with mTOR complex 1 signaling blunted at +1 hour and +48 hours post-training and increased protein degradation markers — though maximal strength gains were not significantly different between groups 23. A 2021 review in Frontiers in Sports and Active Living synthesized these findings with a nuanced message: CWI negatively influences resistance training adaptations but does not appear to blunt endurance training adaptations, making mode-specificity critical in protocol planning 24.

The underlying mechanism is that inflammation and the associated cellular stress signals are not merely damaging byproducts of training — they are essential triggers for the hypertrophic cascade. By attenuating this signaling, CWI interferes with the body's adaptation engine.

Clinical guidance: CWI post-resistance training is best reserved for periods when recovery speed (e.g., daily competition) takes priority over long-term hypertrophy. For general wellness users, recreational athletes, or endurance athletes, this caveat is largely irrelevant.

Mental Health and Mood

Evidence for CWI's positive effects on mood, affect, and mental wellbeing has grown substantially and is now supported by multiple mechanistic and observational lines of evidence.

Acute mood effects are well-documented. A randomized controlled study involving 33 healthy adults found that a single 20°C, 5-minute whole-body immersion significantly increased positive affect (feelings of alertness, activity, attention, inspiration, pride) and decreased negative affect (distress, nervousness) 8. Neuroimaging showed that positive mood changes were associated with altered coupling between brain regions involved in attention control, emotion regulation, and self-regulation — including the medial prefrontal cortex (MPFC) and anterior cingulate cortex (ACC), regions implicated in cognitive flexibility and depressive rumination 8. A separate randomized trial found that cold-water immersion significantly improved scores on the Profile of Mood States (POMS), with a 15-point reduction in the CWI group versus 2 points in controls, with significant improvements in vigour, esteem-related affect, tension, anger, depression, fatigue, and confusion subscales 25.

Depression: A widely cited 2018 case report documented a 24-year-old woman with treatment-resistant major depressive disorder (MDD) who had failed multiple antidepressant medications. A program of weekly supervised open cold-water swimming produced immediate mood improvement after each swim, gradual symptom reduction, and — after one month — reduction in antidepressant medication, with complete cessation at four months. At one-year follow-up she remained medication-free 26. While a single case report cannot establish efficacy, the biological plausibility is strong: proposed mechanisms include catecholamine elevation, anti-inflammatory cytokine modulation (since elevated neuroinflammatory markers are found in many depressed patients), beta-endorphin release, and vagal nerve stimulation 26. A large NHS-funded two-year RCT (the Outside 2 study) is currently underway in the UK to test whether cold-water swimming can serve as a formal adjunct to treatment for mild-to-moderate depression 27.

Dopamine's sustained profile: Unlike the sharp spike-and-crash of stimulant-driven dopamine release, cold-induced dopamine elevation appears characterized by a more gradual rise and sustained duration — several hours by some accounts — consistent with its perceived subjective effects of enhanced motivation, focus, and stable mood rather than agitation [4, 8].

Metabolic Health

CWI's metabolic effects are primarily mediated through three pathways: sympathoadrenal catecholamine release, BAT thermogenesis activation, and skeletal muscle thermogenesis.

Brown adipose tissue and energy expenditure: As detailed in the Mechanisms section, habitual cold exposure recruits BAT volume and enhances cold-induced thermogenesis. Søberg et al. found that experienced winter swimmers burned more calories during cold challenge than controls, and associated data from PET studies place BAT-attributable cold-induced thermogenesis at 120–370 kcal/day in BAT-active individuals [12, 13]. The potential of this mechanism for weight management — while promising — should be understood as a complement to, not replacement for, diet and exercise.

Insulin sensitivity: Evidence is mixed but suggestive. A 2016 study of middle-aged cold-water swimmers over six consecutive months found improved insulin sensitivity in lean subjects (BMI < 25 kg/m²) and in female participants, concluding that "cold water swimming may beneficially modulate insulin sensitivity in cold acclimated lean swimmers" 28. Conversely, a 2025 study of sixteen daily 10-minute sessions at 14°C in healthy non-obese young adults found a transient reduction in insulin sensitivity and glucose tolerance — though these effects reversed to baseline after one week without cold exposure 29. The divergence likely reflects differences in duration (short 10-minute daily sessions vs. months of sustained cold-water swimming), the presence or absence of shivering thermogenesis, and baseline metabolic status. Cold exposure that is sufficient to induce shivering appears to be a key driver of skeletal muscle glucose uptake, since skeletal muscle accounts for approximately 85% of whole-body glucose uptake at insulin-stimulated states 29.

Metabolic rate: At 14°C, metabolic rate increases by 350% during immersion — a dramatic acute effect, though the total caloric burn from a 10–15 minute session is modest in absolute terms 6.

Immune Function

Buijze et al. (2016) conducted the largest randomized controlled trial of cold exposure and immune outcomes to date, involving 3,018 Dutch participants randomized to hot-to-cold showers of 30, 60, or 90 seconds duration or a control (hot shower only) for 30 consecutive days. The primary finding was a 29% reduction in self-reported sick days (sickness absence from work) in the cold shower groups compared to controls (incidence rate ratio: 0.71, p = 0.003). Notably, there was no significant difference in illness days experienced — only in the impact of those days on work attendance — suggesting that cold exposure may reduce the functional severity of illness rather than prevent infection per se. The effect was similar regardless of cold shower duration (30, 60, or 90 seconds), suggesting a threshold rather than dose-response relationship 30. The combination of cold shower and regular physical exercise produced an estimated 54% reduction in sickness absence 30.

The broader 2025 systematic review and meta-analysis of cold-water immersion health effects (11 studies, 3,177 participants) found no significant meta-analytic effect on immune function immediately or at 1 hour post-CWI, but narrative synthesis supported longer-term immune benefits, consistent with Buijze's sickness absence finding 18. Mooventhan and Nivethitha's 2014 systematic review of hydrotherapy across body systems cited evidence that repeated cold water stimulation increased leukocytes, granulocytes, circulating IL-6, natural killer cells and NK cell activity, with daily brief cold stress over 8 days increasing cytotoxic T-lymphocytes and NK cells 31.

Sleep Quality

Post-exercise whole-body CWI appears to improve aspects of sleep quality, likely through its effects on core body temperature. A study comparing whole-body versus partial (legs only) CWI versus no immersion found that whole-body immersion produced a significantly greater proportion of slow-wave sleep (SWS) in the first 180 minutes of the night, reduced N1 sleep (lighter, less restorative stage), fewer arousals, and fewer limb movements — with participants in the whole-body group reporting feeling more refreshed in the morning 32. The mechanism involves the role of core body temperature decline as a cue for sleep initiation; CWI accelerates this decline. The 2025 systematic review found evidence linking CWI to improved sleep outcomes, though the evidence was limited mainly to male subjects 18.

Focus, Alertness, and Cognitive Performance

The catecholamine surge from CWI has direct implications for cognitive state. Norepinephrine increases arousal, vigilance, and attentional capacity via locus coeruleus projections throughout the cortex. Dopamine enhances motivation, cognitive flexibility, and working memory through dopaminergic projections to the prefrontal cortex. Subjective reports of improved mental clarity and focus following cold immersion are consistent with these neurochemical effects, which are documented to persist for several hours post-immersion [4, 8]. The neuroimaging data from Yankouskaya et al. (2023) specifically linked cold-induced positive affect to increased connectivity between the default mode network, salience network, frontoparietal network, and dorsal attention network — a pattern suggesting broad upregulation of attention and executive function circuitry 8.

Cardiovascular Adaptation

In healthy individuals without cardiovascular disease, habitual cold water exposure appears to confer protective cardiovascular adaptations. Studies in cold-adapted populations suggest improvements in lipid profiles, endothelial function, and reductions in cardiovascular risk factor markers 7. The repeated cycle of vasoconstriction and reactive vasodilation functions as a form of "vascular exercise," training blood vessel reactivity. Some research documents lower troponin levels (indicating less cardiac stress) in adapted versus non-adapted cold swimmers 33.

However, the acute cardiovascular effects of CWI are significant stressors: heart rate rises sharply in the first minute of immersion, systolic and diastolic blood pressure both increase (one study found systolic blood pressure reaching 135 mmHg during immersion), and heart rate normalizes or decreases after 15 minutes as the initial shock resolves 34. These acute stresses are manageable for healthy individuals but represent genuine risk for those with uncontrolled hypertension, coronary artery disease, or arrhythmia — addressed in the Safety section.

Protocols

Temperature

The most commonly used wellness temperature range is 10–15°C (50–59°F). This range is cold enough to reliably trigger the catecholamine and thermoregulatory responses but sufficiently above freezing to allow sessions of 5–15 minutes without extreme risk. Key temperature benchmarks from research:

• 14°C (57°F): The temperature used in the Šrámek study that documented 530% noradrenaline increase 6; also the temperature in the Eimonte et al. norepinephrine study 7
• 10°C (50°F): Temperature used in Roberts et al.'s recovery and hypertrophy studies 22
• ~15°C (59°F): The lower end of the Buijze cold shower protocol 30
• 20°C (68°F): Used in the Yankouskaya brain connectivity study; still produced meaningful mood and catecholamine effects 8

Below 10°C increases physiological risk without proportionate additional benefit for most wellness users, and significantly shortens the tolerable exposure window. The Frontiers meta-analysis found the 11–15°C range most effective for DOMS reduction 20.

Duration

The optimal duration for DOMS reduction in the network meta-analysis was 10–15 minutes — a range that may be impractical for beginners and is longer than needed for neurochemical or mood effects 20.

Weekly Dose

Andrew Huberman, synthesizing the available literature, codified the concept of an 11-minute weekly minimum effective dose — distributed across 2–4 sessions of 1–5 minutes each rather than in a single session 4. This figure reflects data from studies that found meaningful thermoregulatory and metabolic effects in subjects completing multiple brief weekly immersions, consistent with the Søberg et al. finding that experienced winter swimmers performed their practice 2–3 times per week 12. The principle of distribution (multiple shorter sessions) appears more effective than equivalent time consolidated into one session, both because acute catecholamine responses reset between sessions and because repeated cold exposure — not a single prolonged one — drives BAT recruitment and HRV adaptation.

Basic Protocol Structure

1. Before entering: Take several slow, controlled breaths. Nasal breathing is preferable when possible to limit hyperventilation. Accept that an initial gasp is normal.

2. Entry: Enter gradually (feet first) or rapidly depending on personal preference and facility design. Both are used clinically.

3. During immersion: Breathe slowly and deliberately. The initial hyperventilation phase (3–5 minutes) diminishes with practice. Immersion to the neck — including hands and feet — produces the most robust physiological response 4. Focus on relaxing the body and allowing the cold to be present without fighting it.

4. Exit: Exit before shivering becomes uncontrollable or skin color becomes blue or mottled. After exiting, allow the body to rewarm naturally rather than immediately showering with hot water. Natural rewarming (including mild shivering) enhances the metabolic effect and prolongs catecholamine elevation 4.

5. Do not towel off aggressively immediately — retaining skin moisture slightly prolongs the evaporative cooling effect and the body's thermogenic response.

Hot-Cold Cycling

The Scandinavian practice of alternating hot sauna with cold plunge is well-supported. The Søberg study specifically examined winter swimmers who combined cold immersion with sauna 12. When cycling, conventional guidance (consistent with Huberman's synthesis of the literature) is to end with cold when the goal is stimulation, alertness, and catecholamine elevation 4. Ending with sauna/heat is more appropriate when the goal is relaxation and sleep preparation.

Timing and Training Considerations

The training timing of cold water immersion is one of the most practically important — and most misunderstood — aspects of the modality. The relevant variables are: (a) goal orientation (recovery/resilience vs. hypertrophy); (b) training modality (resistance vs. endurance); and (c) time of day.

Pre-Workout Timing

Cold immersion before training can enhance sympathetic activation, alertness, and mental readiness — potentially useful as a pre-competition arousal protocol or when training demands cognitive sharpness. However, peripheral cooling reduces muscle temperature and decreases force production capacity and neuromuscular efficiency in the short term. For power and strength athletes, pre-training cold immersion is generally not recommended.

Post-Resistance Training

As detailed in the Benefits section, regular post-resistance-training CWI blunts hypertrophy signaling and should be avoided when building muscle mass is the priority [22, 23]. The evidence suggests:

• If hypertrophy is the goal: Do not use CWI within 4–6 hours post-resistance training. Active recovery (light cycling, walking) is a better choice for this window 22.
• If recovery speed is the priority (e.g., multiple training sessions per day, tournament play, back-to-back competition days): CWI's accelerated recovery of soreness and neuromuscular function is well worth the blunted hypertrophic signal 24.
• For endurance athletes: CWI after endurance training does not appear to blunt adaptations and may enhance recovery and mitochondrial biogenesis markers 24.

Post-Endurance Training

Several studies indicate that post-endurance-exercise CWI may actually augment adaptations by stimulating PGC-1α-mediated mitochondrial biogenesis pathways, consistent with findings that CWI amplifies markers of aerobic adaptation 24. This makes CWI a stronger fit for runners, cyclists, and triathletes than for bodybuilders.

Time of Day

The sustained catecholamine and dopamine elevation from CWI creates a state of alertness that can interfere with sleep if the session occurs too close to bedtime. Morning cold exposure (within 1–2 hours of waking) aligns naturally with the body's cortisol awakening response and sympathetic peak, amplifying the natural energy-generating effects. Evening use is appropriate for some practitioners and may promote slow-wave sleep via core temperature drop, but immersion should ideally be completed at least 2–3 hours before intended sleep onset to allow catecholamine levels to normalize.

Safety and Contraindications

Cold water immersion is a potent physiological stressor. For healthy individuals following appropriate protocols, it is well-tolerated and generally safe. For specific populations and conditions, it carries genuine risk.

Cold Shock Response and Drowning Risk

The initial cold shock — particularly in open-water settings — is the most acutely dangerous phase. The involuntary gasp reflex, if the head is submerged, can result in immediate aspiration and drowning. Controlled, supervised immersion (as in a cold plunge pool) essentially eliminates this risk. The cold shock response habituates with repeated exposure: systematic review data suggest meaningful attenuation after approximately 4 immersions, with progressive further reduction thereafter 35. This habituation represents one of the strongest arguments for gradual acclimation protocols in new users.

Cardiac Risk: Arrhythmia and Autonomic Conflict

Cold water immersion creates "autonomic conflict": the cold shock response drives sympathetic tachycardia while facial immersion and breath-holding activate the vagal diving reflex (parasympathetic bradycardia). Simultaneous strong activation of both autonomic divisions significantly increases arrhythmia risk. Tipton et al. found arrhythmias in approximately 2% of head-out cold water immersions in young, healthy laboratory subjects, rising dramatically (62–82%) when breath-holding was combined with cold water submersion 36. Most arrhythmias in healthy individuals are supraventricular and hemodynamically effective, but in the presence of pre-existing cardiac pathology, the risk of life-threatening ventricular arrhythmia is elevated 36.

Contraindications include:

• Atrial fibrillation or known arrhythmia: Cold and associated catecholamine surge can precipitate episodes 33
• Uncontrolled hypertension: Acute blood pressure elevation (up to 135/81 mmHg in research settings) is contraindicated in inadequately controlled hypertensives 34
• Coronary artery disease / history of MI: The increased cardiac workload, coronary vasospasm risk (especially with sauna alternation), and sympathetic surge represent genuine risk 33
• History of cardiac arrest or long QT syndrome: Absolute contraindication

Raynaud's Phenomenon

Raynaud's disease involves exaggerated vasoconstriction of the digital vessels in response to cold, producing painful color changes (white → blue → red) in the fingers and toes. Cold plunge would predictably provoke severe Raynaud's episodes and is contraindicated in affected individuals. Peripheral artery disease similarly represents a contraindication 33.

Asthma and Cold-Induced Bronchospasm

Cold, dry air inhalation is a well-recognized trigger for exercise-induced bronchoconstriction (EIB) and asthma exacerbations. Cold water immersion involves breathing cold, humid air rather than cold dry air, which is somewhat less bronchoconstricting than cold dry air. However, the hyperventilation phase of the cold shock response — with high minute volumes — can still dry and cool the airways sufficiently to trigger bronchoconstriction in susceptible individuals 37. Individuals with asthma or EIB should consult their pulmonologist before beginning CWI, ensure rescue bronchodilators are immediately available, and begin with shorter, less thermally extreme sessions.

Pregnancy

Cold water immersion during pregnancy is not recommended. Vasoconstriction responses may reduce uteroplacental blood flow and oxygen delivery to the fetus. Limited research raises concerns about association between cold exposure in early pregnancy and elevated preterm birth risk, though evidence is insufficient for definitive conclusions 38. The body's altered thermoregulatory set-point and reduced HPA axis responsiveness during pregnancy also increase the risk of hypothermia. The current consensus is that the potential risks to the fetus outweigh any benefits to the mother, and CWI should be avoided throughout pregnancy 38.

Hypothermia

Hypothermia (core body temperature < 35°C) is unlikely in a supervised, time-limited immersion in the typical wellness range (10–15°C for 5–15 minutes), but can occur with prolonged exposure or in already-cold individuals. Elderly individuals, those with low body mass, and those on beta-blockers or other medications affecting thermoregulation are at elevated risk. Warning signs — confusion, severe shivering, mottled or blue-grey skin — indicate immediate exit and active warming.

Contraindication Summary

Summary and Integration

Cold water immersion is one of the most physiologically potent wellness modalities available, with a mechanistic profile that intersects neuroscience, metabolism, immunology, and recovery biology. At appropriate doses, it reliably produces sustained catecholamine elevation (530% noradrenaline increase at 14°C), recruits brown adipose thermogenesis, modulates inflammatory signaling, enhances HRV and vagal tone, and elevates mood — effects that are not merely anecdotal but documented in peer-reviewed literature extending from basic physiology to randomized controlled trials.

The practical integration of CWI into a wellness program requires matching protocol to goal:

• For recovery: 10–15 minutes at 11–15°C post-exercise (avoiding immediately post-resistance training if hypertrophy is the goal)
• For mood, focus, and neurochemical benefits: 2–5 minutes at 10–15°C, ideally in the morning, 2–4 times per week (minimum 11 minutes total weekly)
• For metabolic benefits and BAT activation: Consistent practice over weeks to months, which drives BAT recruitment and improved thermogenic capacity
• For immune resilience: Even 30–60 seconds of cold water exposure (as in hot-to-cold shower protocol) 30 consecutive days produced 29% fewer sick days in the Buijze trial

Safety screening is non-negotiable. The acute cardiovascular stress of CWI — while well-managed by healthy individuals — is contraindicated in cardiac, vascular, and several other clinical populations. A brief health intake and physician clearance for at-risk clients represents both ethical obligation and sound clinical practice.

Physiological Phases of Immersion

A complete cold water immersion session can be divided into three physiologically distinct phases, each with its own risk profile and therapeutic window. Understanding these phases helps practitioners guide clients through the experience effectively.

Phase 1: Cold Shock (0–3 Minutes)

Upon contact with cold water, the skin thermoreceptors — particularly the cold-sensitive C-fiber and Aδ-fiber afferents — fire at extremely high rates, triggering the full cold shock cascade: inspiratory gasp (transient apnea risk), uncontrolled hyperventilation (minute ventilation can rise to 40–60 L/min), tachycardia, and acute hypertension 5. Peripheral vasoconstriction begins immediately. The sympathoadrenal surge is most intense during this phase — the catecholamine elevation documented by Šrámek et al. is primarily an acute Phase 1 phenomenon 6.

For beginners, this is the phase that feels overwhelming. The appropriate coaching response is to prepare the practitioner to expect the gasp and hyperventilation — normalizing the sensation dramatically reduces anxiety and improves tolerance. Slow nasal exhalation during this phase counteracts hyperventilation and reduces the risk of syncope from the accompanying respiratory alkalosis (CO₂ washout). The cold shock response habituates significantly after just 4 exposures 35, which is why structured progressive programs that begin with shorter exposures and build gradually produce dramatically better long-term adherence and comfort than single-session immersion.

Phase 2: Short-Term Acclimation (3–15 Minutes)

After the initial sympathetic peak subsides, the body transitions into a more stable thermoregulatory response. Shivering thermogenesis is activated in skeletal muscle, supplemented by BAT non-shivering thermogenesis 10. Peripheral vasoconstriction continues to restrict cutaneous heat loss. Heart rate, which spikes in Phase 1, normalizes and may decrease below baseline — partly from the combined effect of cold-induced cardiac slowing and parasympathetic reactivation 34. HRV begins to increase. The subjective experience typically shifts from acute stress to something practitioners describe as a "focused calm" — consistent with the parasympathetic shift that characterizes this phase.

This phase is where most of the therapeutic mechanisms described in the Mechanisms section — catecholamine signaling, lymphatic pumping, cold shock protein induction, and metabolic activation — are active and sustained. The majority of wellness CWI protocols (2–15 minutes) operate entirely within this phase.

Phase 3: Cooling and Hypothermia Risk (Beyond 15–20 Minutes)

With extended immersion, core body temperature begins to fall. The muscle cooling that characterizes this phase reduces neuromuscular efficiency — grip strength, swimming ability, and coordination all decline, elevating drowning risk in open-water settings. Thinking may become impaired. At core temperatures below 35°C, hypothermia is developing. For supervised wellness CWI in a controlled pool environment, Phase 3 is rarely encountered. It is, however, the relevant phase for understanding cold-water drowning incidents, which are why duration limits and supervision are non-negotiable safety requirements in any CWI program 5.

Mechanisms of Action: Deeper Synthesis

Mitochondrial Biogenesis

Cold exposure and the associated catecholamine surge activate PGC-1α (peroxisome proliferator-activated receptor gamma coactivator 1-alpha), the master regulator of mitochondrial biogenesis. This pathway — shared with endurance exercise — is one reason researchers have observed that post-endurance-exercise CWI may amplify aerobic adaptations 24. In brown adipose tissue specifically, the cold-driven upregulation of UCP1, mitochondrial content, and oxidative capacity constitutes a genuine form of mitochondrial proliferation 10. In skeletal muscle, multiple studies document increased markers of mitochondrial biogenesis with habitual cold exposure, including increased citrate synthase activity and respiratory chain protein content 24. These adaptations collectively improve metabolic efficiency, increase the proportion of slow-twitch oxidative muscle fibers over time, and may have implications for age-related metabolic decline.

Endorphin and Serotonin Release

Beyond the catecholamine axis, cold water immersion stimulates release of β-endorphins (endogenous opioids) and serotonin 31. β-endorphins bind to μ-opioid receptors and contribute to the post-immersion analgesia, euphoria, and well-being that many practitioners report. Serotonin, the neurotransmitter deficiency of which is implicated in depression, anxiety, and emotional dysregulation, is synthesized and released in response to cold. Vagal nerve stimulation — triggered by cold facial immersion via the diving reflex — has direct anti-inflammatory effects and is also hypothesized to enhance serotonin signaling in limbic circuits 26. The full neurochemical profile of a cold plunge session thus includes norepinephrine, dopamine, epinephrine, β-endorphin, and serotonin — a combination that is difficult to replicate with any single pharmacological intervention.

The Wim Hof Method: Evidence Profile

The Wim Hof Method (WHM) deserves specific attention because it is the most widely practiced and scientifically studied cold-exposure system in the modern era. The method integrates three components: cold exposure (ice baths, cold showers, winter swimming), specific breathing exercises (cyclic hyperventilation followed by breath retention), and meditation/concentration techniques.

The 2014 Kox et al. PNAS study remains the most significant scientific documentation of the WHM's effects. In a controlled endotoxemia challenge (intravenous injection of bacterial endotoxin lipopolysaccharide), WHM-trained participants demonstrated: profound epinephrine increases (up to 300% over baseline) triggered by the breathing exercises before LPS administration; proinflammatory cytokines (TNF-α, IL-6, IL-8) that were approximately 50% lower than control subjects; anti-inflammatory IL-10 approximately 200% higher; and significantly reduced flu-like symptoms [3, 46]. The study's conclusion — that the sympathetic nervous system and innate immune response can be voluntarily influenced via a short-term training program — was paradigm-shifting. Prior to this work, both systems were considered fully involuntary.

Follow-up research has applied WHM in patients with axial spondyloarthritis, a chronic inflammatory condition, finding significant declines in the inflammatory markers ESR and CRP, improvements in disease activity scores, and improvements in quality of life — suggesting potential clinical relevance for immune-mediated inflammatory diseases 45. The breathing component of WHM appears to be the primary driver of the acute immune modulation (via rapid epinephrine release), while the cold exposure contributes to longer-term autonomic and inflammatory adaptations.

Practical Guidance for Practitioners

Client Intake Assessment

Before introducing any client to CWI, a practitioner should screen for the following:

• Cardiovascular history: Arrhythmia, atrial fibrillation, MI, coronary artery disease, congestive heart failure, uncontrolled hypertension
• Peripheral vascular disease or Raynaud's syndrome
• Pulmonary disease: Asthma, COPD, significant reactive airway disease
• Metabolic conditions: Poorly controlled diabetes (hypoglycemia risk from altered glucose dynamics)
• Neurological conditions: Epilepsy (cold can be a seizure trigger in rare cases)
• Pregnancy
• Current medications: Beta-blockers (impair heart rate response and thermoregulation), calcium channel blockers, diuretics, and insulin

For clients with any positive history in these areas, physician clearance should be obtained before participation.

Progressive Acclimation Program

A well-structured 4-week onboarding program substantially reduces both psychological barrier and physiological risk:

After 4 sessions, the cold shock response is meaningfully attenuated 35. After 2–4 weeks of consistent practice, most healthy individuals are comfortable with the standard 10–15°C range for 5–10 minutes.

Contraindication Table: Expanded

Additional Evidence Notes

Several additional studies and reviews provide supporting context for the mechanisms and benefits described above:

Cardiovascular and autonomic: A systematic review of voluntary cold-water exposure published in International Journal of Environmental Research and Public Health (2022) surveyed the health effects of both recreational cold-water swimming and structured cold plunge protocols, documenting improvements in cardiovascular markers, mood, and pain thresholds in habituated practitioners 41. A study examining cardiac-vagal activation after cold stimulation found that brief cold intervals applied to the skin were sufficient to measurably accelerate parasympathetic reactivation, consistent with the HRV data in athlete recovery studies 42.

Inflammatory cytokine kinetics: Pilot data from a water immersion IL-6 study found that cold water immersion (versus thermoneutral water) uniquely maintained or slightly elevated serum IL-6 in the 90 minutes following post-exercise immersion — a finding consistent with the acute pro-inflammatory then delayed anti-inflammatory temporal pattern described in the 2025 systematic review 43. Elevated circulating IL-6 post-exercise, when it is not pathological, is now understood to act as a myokine signaling metabolic adaptation and fat oxidation — a nuance that complicates the simple "anti-inflammatory" narrative.

Cold exposure and metabolic disease: A narrative review specifically examining cold exposure as a potential therapeutic strategy for metabolic disease found that repeated cold exposures can lower fasting glucose and insulin in individuals with type 2 diabetes, and that both acute and repeated cold exposures improved insulin sensitivity — with the caveats that critical gaps remain and that much of the benefit appears attributable to shivering skeletal muscle thermogenesis rather than BAT alone 39.

Brown adipose tissue communication: Emerging research into BAT-brain communication pathways (via afferent signals from BAT thermoreceptors and adipokines) proposes that BAT does not merely generate heat but also sends signals that influence feeding behavior, circadian rhythmicity, and potentially mood — pathways that may partly explain why the circadian temperature peak seen in experienced winter swimmers (Søberg et al.) has implications beyond thermoregulation 44.

Sea swimming for depression: A case series and theoretical framework published in the Journal of Affective Disorders expanded on the open-water swimming depression evidence, proposing a "cold water swim therapy" model that integrates cross-adaptation to stress, catecholamine responses, and the social/environmental benefits of open-water swimming 48. The British Heart Foundation has separately summarized the emerging evidence and risk profile for cold-water swimming, noting that while it is contraindicated for those with cardiac conditions, habituated healthy swimmers appear to derive cardiovascular benefits 49.

Nrf2 as master antioxidant regulator: A 2022 comprehensive review of the Nrf2-Keap1 pathway confirmed its role as the primary cellular defense against oxidative and electrophilic stress — the same pathway that acute cold exposure transiently activates in a hormetic fashion — with downstream protection against a wide range of chronic diseases including cardiovascular disease, neurodegeneration, and metabolic syndrome 50. The link between cold-induced hormesis and Nrf2 activation positions CWI as part of a broader class of "positive stressors" alongside intermittent fasting and exercise that maintain cellular resilience through periodic pathway activation 47.

References

[1]: https://pmc.ncbi.nlm.nih.gov/articles/PMC5535692/

[2]: https://www.mehanainstitute.com/hydrotherapy/

[3]: https://www.pnas.org/doi/10.1073/pnas.1322174111

[4]: https://www.hubermanlab.com/newsletter/the-science-and-use-of-cold-exposure-for-health-and-performance

[5]: https://en.wikipedia.org/wiki/Cold_shock_response

[6]: https://pubmed.ncbi.nlm.nih.gov/10751106/

[7]: https://www.psychiatrypodcast.com/psychiatry-psychotherapy-podcast/episode-232-cold-exposure-for-mental-health-benefits

[8]: https://pmc.ncbi.nlm.nih.gov/articles/PMC9953392/

[9]: https://pubmed.ncbi.nlm.nih.gov/9884790/

[10]: https://pmc.ncbi.nlm.nih.gov/articles/PMC11989373/

[11]: https://pmc.ncbi.nlm.nih.gov/articles/PMC3726172/

[12]: https://pubmed.ncbi.nlm.nih.gov/34755128/

[13]: https://pmc.ncbi.nlm.nih.gov/articles/PMC12010580/

[14]: https://pmc.ncbi.nlm.nih.gov/articles/PMC5021741/

[15]: https://www.bodyspec.com/blog/post/the_incredible_benefits_of_cold_shock_protein

[16]: https://pmc.ncbi.nlm.nih.gov/articles/PMC8884887/

[17]: https://pmc.ncbi.nlm.nih.gov/articles/PMC5815212/

[18]: https://journals.plos.org/plosone/article?id=10.1371%2Fjournal.pone.0317615

[19]: https://www.biorxiv.org/content/10.1101/2025.09.29.679254v1

[20]: https://www.frontiersin.org/journals/physiology/articles/10.3389/fphys.2025.1525726/full

[21]: https://pmc.ncbi.nlm.nih.gov/articles/PMC12851776/

[22]: https://pmc.ncbi.nlm.nih.gov/articles/PMC4594298/

[23]: https://pubmed.ncbi.nlm.nih.gov/31513450/

[24]: https://www.frontiersin.org/journals/sports-and-active-living/articles/10.3389/fspor.2021.660291/full

[25]: https://onlinelibrary.wiley.com/doi/full/10.1002/lim2.53

[26]: https://pmc.ncbi.nlm.nih.gov/articles/PMC6112379/

[27]: https://www.bbc.com/news/articles/cn8vddjdddpo

[28]: https://pubmed.ncbi.nlm.nih.gov/26966319/

[29]: https://www.sciencedirect.com/science/article/abs/pii/S0306456525000452

[30]: https://pmc.ncbi.nlm.nih.gov/articles/PMC5025014/

[31]: https://pmc.ncbi.nlm.nih.gov/articles/PMC4049052/

[32]: https://pmc.ncbi.nlm.nih.gov/articles/PMC8044518/

[33]: https://www.health.harvard.edu/heart-health/cold-plunges-healthy-or-harmful-for-your-heart

[34]: https://pmc.ncbi.nlm.nih.gov/articles/PMC10842018/

[35]: https://pubmed.ncbi.nlm.nih.gov/38211547/

[36]: https://pmc.ncbi.nlm.nih.gov/articles/PMC3459038/

[37]: https://www.ncbi.nlm.nih.gov/books/NBK557554/

[38]: https://miraclecord.com/news/ice-bath-while-pregnant/

[39]: https://pubmed.ncbi.nlm.nih.gov/33764169/

[40]: https://www.frontiersin.org/journals/immunology/articles/10.3389/fimmu.2022.936689/full

[41]: https://pmc.ncbi.nlm.nih.gov/articles/PMC9518606/

[42]: https://pmc.ncbi.nlm.nih.gov/articles/PMC6334714/

[43]: https://pmc.ncbi.nlm.nih.gov/articles/PMC3499890/

[44]: https://www.eurekalert.org/news-releases/930872

[45]: https://www.wimhofmethod.com/science

[46]: https://pubmed.ncbi.nlm.nih.gov/24799686/

[47]: https://pmc.ncbi.nlm.nih.gov/articles/PMC8322530/

[48]: https://www.sciencedirect.com/science/article/abs/pii/S1755296622000345

[49]: https://www.bhf.org.uk/informationsupport/heart-matters-magazine/activity/cold-water-swimming

[50]: https://pmc.ncbi.nlm.nih.gov/articles/PMC9774434/