Does cold really strengthen the immune system? What hormesis means
Cold is a brief stress stimulus, not a permanent state. That is the whole difference between a training impulse and an additional load. Here I separate the marketing from the physiology.
Cold is neither a miracle cure nor nonsense. It is a dose. And with a dose, the amount decides whether something is training or damage.
You know those videos. Someone climbs into a barrel of ice cubes, breathes loudly, smiles bravely. Underneath is a sentence about the immune system.
And then you sit there, maybe in your fourth infection of this winter, and think: should I be doing that too?
I get this question often. Usually in exactly this mixture of curiosity and quiet scepticism. And I find both justified.
Because there really is something to it. Just rarely the thing written in the caption.
Cold is a stimulus. And in biology, stimuli have a property that regularly gets left out in marketing: their effect is not linear. A little can set something in motion. A lot can break the same thing. There is a name for this principle, and that is what this article is about.
What this article covers
- What hormesis means and why the curve is biphasic
- What happens in the body during the first 60 seconds of cold
- Why norepinephrine rises and epinephrine does not
- Brown adipose tissue and cold acclimatisation
- What the cold shower trial with 3018 people shows and what it does not
- Why the breathing technique is not the cold effect
- When cold becomes counterproductive: infection, exhaustion, chronic stress
- Limits and dangers, from cold shock to cold urticaria
Hormesis: why a small stimulus is something different from a large load
Imagine two people lifting weights. One trains three times a week for an hour. The other trains four hours every day and sleeps five hours.
Both apply the same stimulus. Just with a different outcome.
That is exactly what hormesis describes. The term comes from toxicology and refers to a biphasic dose response relationship. Small dose: stimulating, promoting adaptation. Large dose: inhibiting, damaging. A curve that first goes up and then goes down.
Who: Mattson summarised in 2008 in Ageing Research Reviews what hormesis means in biology and medicine.
What he describes: Hormesis is the adaptive reaction of cells and organisms to moderate, usually time-limited stress. As examples he names ischemic preconditioning, physical training, dietary restriction and low doses of certain plant compounds. At the cellular level, kinases, deacetylases and transcription factors such as Nrf-2 and NF-kappaB are involved. The cell then increases the production of protective and restorative proteins, among them antioxidant enzymes and chaperones.
What that means for you: The benefit does not come from the stimulus itself. It comes from the response the body builds afterwards. And that response needs time and rest.
Mattson. Ageing Res Rev. 2008;7(1):1-7. DOI: 10.1016/j.arr.2007.08.007Who: Calabrese and Mattson re-framed the hormesis concept for biology, toxicology and medicine in 2017 in NPJ Aging and Mechanisms of Disease.
What they state: They describe hormesis as an adaptive response of biological systems to moderate challenges, through which the system improves its functional capacity or its tolerance towards stronger loads. In the analysed datasets, the typical stimulatory range lies roughly 30 to 60 percent above the control value. Terms such as U-shaped or biphasic dose response curve are used largely interchangeably in the literature.
What that means for you: The effect size of hormesis is modest. It is a matter of percentages, not of transformation. Anyone expecting a doubling has misunderstood the concept.
Calabrese, Mattson. NPJ Aging Mech Dis. 2017;3:13. DOI: 10.1038/s41514-017-0013-zNothing happens
The body receives no signal that it would need to adapt anything. Comfort without information.
Adaptation arises
Short, distinct, followed by recovery. The body builds up protective and repair systems. That is the hormetic range.
Load instead of training
Too long, too frequent, too cold or without recovery. The impulse becomes an additional load for a system that is already working.
The decisive question with cold is not: how much can I endure?
The decisive question is: how small may the stimulus be for my body still to respond? With hormesis, toughness does not win. Dosing wins.
And now you know why, with cold, I talk about dose and not about courage.
The first 60 seconds: what cold sets off in your body
You step into cold water. Your body reacts before you can make a decision.
This first phase is called the cold shock response. It is not weakness and not a lack of training. It is a reflex, triggered via the cold receptors of the skin.
Skin receptors fire
The sudden drop in skin temperature stimulates the peripheral cold receptors massively. The signal goes directly into the autonomic nervous system.
Gasp and hyperventilation
An involuntary deep inhalation, then a barely controllable rapid breathing. It typically lasts two to three minutes.
Sympathetic nervous system and catecholamines
Heart rate and blood pressure rise, the skin vessels constrict. In the blood it is above all norepinephrine that rises.
Shivering and thermogenesis
The muscles begin to shiver, energy expenditure rises markedly. The body produces heat to hold its core temperature.
Afterwards: the adaptation phase
This is where the actual hormetic effect arises. Not in the water, but in the hours afterwards, when the body responds to the stimulus.
Who: Šrámek and colleagues studied young men in 2000 in the European Journal of Applied Physiology during one-hour immersions in water of 32, 20 and 14 degrees.
What they measured: At 14 degrees the plasma norepinephrine concentration rose by 530 percent and dopamine by 250 percent. Energy expenditure rose by 350 percent. Heart rate and systolic as well as diastolic blood pressure rose by 5, 7 and 8 percent. Epinephrine, by contrast, remained unchanged, and cortisol showed a tendency to fall. The authors concluded that the cold-induced reactions are mediated predominantly via increased activity of the sympathetic nervous system.
What that means for you: The often quoted image of an adrenaline rush does not hold up physiologically. Cold is above all a norepinephrine story. And norepinephrine is more the messenger of alertness and focus than the messenger of panic.
Šrámek, Šimečková, Janský, Šavlíková, Vybíral. Eur J Appl Physiol. 2000;81(5):436-442. DOI: 10.1007/s004210050065That explains well, by the way, why people feel awake and clear after a cold shower. This effect is real and it is immediate. It is just not an immune finding. It is a catecholamine response.
Established: cold water immersion raises norepinephrine, energy expenditure, heart rate and blood pressure measurably. Established: this reaction is mediated via the sympathetic nervous system. Not established: that direct immune protection in healthy people can be derived from it. That is a different step, and so far it has not been cleanly demonstrated.
And now you know why I speak of a norepinephrine kick and not of an immune boost.
Adaptation: your body learns cold surprisingly fast
This is where it gets really interesting for me. Because if hormesis holds, the adaptation ought to be measurable.
It is. And very clearly so.
Who: Tipton, Golden and colleagues examined in 1998 in Portsmouth how quickly the cold shock response can be attenuated. Thirteen healthy men were divided into a control group and a habituation group.
What they observed: The habituation group completed six immersions of three minutes each in water at 15 degrees. Afterwards, at 10 degrees, the respiratory rate in the first 30 seconds fell from 47.3 to 24.0 breaths per minute, the minute ventilation from 67.6 to 29.5 litres per minute and the heart rate from 128 to 109 beats per minute. In the control group nothing changed.
What that means for you: Six short sessions were enough. And the habituation was achieved in water warmer than the water that had to be endured afterwards. So the stimulus does not have to be maximal at all.
Tipton, Golden, Higenbottam, Mekjavic, Eglin. Eur J Appl Physiol Occup Physiol. 1998;78(3):253-257. DOI: 10.1007/s004210050416Who: The same research group around Tipton, Mekjavic and Eglin examined in 2000 how long this habituation lasts. The participants were retested repeatedly over 14 months.
What they found: Seven months after the habituation block, respiratory rate, minute ventilation and heart rate in the first 30 seconds were still significantly lower than on the first day. After 14 months the heart rate was still dampened, while the breathing values had moved back towards baseline.
What that means for you: An adaptation once acquired is remarkably long lasting. That is hormesis in its most visible form: a short stimulus, an adaptation that carries over months.
Tipton, Mekjavic, Eglin. Eur J Appl Physiol. 2000;83(1):17-21. DOI: 10.1007/s004210000255That is the most honest finding on the subject of cold. What measurably improves is your reaction to cold. Not your defence against colds. These two things are constantly confused.
And now you know why I describe cold training first of all as nervous system training.
Brown adipose tissue: the metabolic part of the story
There is a second adaptation that looks less spectacular and interests me more clinically.
For a long time brown adipose tissue was regarded as something only infants and small mammals have. Brown fat burns energy directly into heat instead of storing it. In adults, so the teaching went, nothing relevant was left of it.
That assumption has fallen.
Who: van Marken Lichtenbelt and colleagues examined 24 healthy men in 2009 in the New England Journal of Medicine, ten lean and fourteen overweight, with combined PET and CT imaging.
What they found: Under mild cold of 16 degrees, activity of brown adipose tissue appeared in 23 of the 24 participants, that is in 96 percent. Under thermoneutral conditions at 22 degrees this activity was not found. In the overweight participants it was significantly lower than in the lean ones, with p equal to 0.007.
What that means for you: Adult humans have brown adipose tissue. It is activated by cold. And it appears to be less active with a higher body fat percentage.
van Marken Lichtenbelt, Vanhommerig, Smulders et al. N Engl J Med. 2009;360(15):1500-1508. DOI: 10.1056/NEJMoa0808718Who: Yoneshiro and colleagues examined in 2013 in the Journal of Clinical Investigation whether brown adipose tissue can be built up again in people with low baseline activity.
What they observed: An acute cold exposure at 19 degrees over two hours increased energy expenditure. Over six weeks of two hours daily at 17 degrees, the activity of brown adipose tissue and cold-induced thermogenesis rose in parallel, while body fat mass decreased. The changes in fat mass and in adipose tissue activity were negatively correlated.
What that means for you: Cold acclimatisation is not all or nothing. It can be trained. Important for placing this: here it was two hours daily over six weeks at mild cold, not 60 seconds in an ice barrel.
Yoneshiro, Aita, Matsushita et al. J Clin Invest. 2013;123(8):3404-3408. DOI: 10.1172/JCI67803From the perspective of clinical psychoneuroimmunology, which I work with, this is the more interesting track. Not the defence against viruses, but metabolic flexibility. That is, your body's ability to switch between operating states instead of being stuck in one.
Who: Esperland, de Weerd and Mercer reviewed the literature on voluntary cold water exposure in 2022 in the International Journal of Circumpolar Health. After the filtering process, 104 papers remained.
What they state: Cold water immersion appears to reduce or convert adipose tissue, to lower insulin resistance and to improve insulin sensitivity. That could have a protective effect in the direction of cardiovascular and metabolic disease. At the same time they are clearly critical: small groups, often only one sex, differing temperatures and salt contents. Whether winter swimmers as a group are inherently healthier is unclear, they write.
What that means for you: The last sentence is the most important one. People who voluntarily step into ice water are not a random sample. They are usually more mobile, more motivated and healthier than average. This is called self-selection, and it distorts almost every observational study on this subject.
Esperland, de Weerd, Mercer. Int J Circumpolar Health. 2022;81(1):2111789. DOI: 10.1080/22423982.2022.2111789And now you know why, with cold, I look at metabolism first and only afterwards at immune defence.
The immune system: what the studies show and what they do not
Now to the core question. And I want to be very precise here, because this is the point where most articles become sloppy.
There is exactly one large randomised human trial on cold and illness. It is well done and it is constantly cited incorrectly.
Who: Buijze and colleagues randomised 3018 adults between 18 and 65 years between January and March 2015, without serious pre-existing conditions and without cold shower experience. The paper was published in 2016 in PLOS ONE.
What they found: The four groups showered cold for 30, 60 or 90 seconds at the end of each shower for 30 days, or not at all. 79 percent completed the protocol. The analysis showed a reduction of self-reported days of sickness absence by 29 percent, with an incidence rate ratio of 0.71 and p equal to 0.003. For the number of sick days itself there was no significant group effect. Serious adverse events did not occur.
What that means for you: So the participants were ill roughly equally often. They simply called in sick less often. That is an interesting finding, but it is something different from strengthened immune defence.
Buijze, Sierevelt, van der Heijden, Dijkgraaf, Frings-Dresen. PLoS One. 2016;11(9):e0161749. DOI: 10.1371/journal.pone.0161749It was not blinded. Anyone taking cold showers knows it. Both endpoints were self-reported, without medical confirmation. And the duration of the cold shower made no discernible difference, 30 seconds performed similarly to 90.
That sickness absences fell while sick days did not can be read in at least two ways. Either the infections ran a milder course. Or the participants felt more resilient and were more likely to go to work. Both are possible. The study cannot distinguish between them.
What happens immediately after the cold
You might expect immune markers to shoot up right after an ice bath. The data are less consistent than that.
Who: Janský and colleagues examined the immune system of cold-exposed and cold-adapted people in 1996 in the European Journal of Applied Physiology and Occupational Physiology.
What they observed: A single one-hour immersion at 14 degrees had only minimal effects immediately afterwards. After six weeks with three immersions per week, a small but significant rise appeared in the monocyte proportion, in lymphocytes carrying the IL-2 receptor and in plasma TNF-alpha. Unchanged were, among others, IL-1-beta, CRP, the immunoglobulins and the total count of leukocytes and granulocytes. The authors wrote verbatim that the biological significance of the observed changes still had to be clarified.
What that means for you: Small shifts in laboratory values are not the same as better protection against infections. That gap has not been closed to this day.
Janský, Pospíšilová, Honzová et al. Eur J Appl Physiol Occup Physiol. 1996;72(5-6):445-450. DOI: 10.1007/BF00242274Who: Eimonte and colleagues examined in 2021 in the International Journal of Hyperthermia what happens in the following 48 hours after a short whole-body immersion at 14 degrees for ten minutes, in young men who were not cold adapted.
What they measured: The cold produced a clear stress reaction with hyperventilation, muscle shivering, increased heat production and heart rate as well as a strong release of epinephrine, norepinephrine and cortisol. The rise in IL-6 was delayed, TNF-alpha production was reduced, IL-1-beta remained unchanged. The neutrophil proportion rose with a delay, the lymphocyte proportion fell.
What that means for you: Here too the most important sentence of the paper applies: within 48 hours the participants showed no increased tendency towards cold symptoms. So cold apparently does not make you ill. But the paper shows just as little that it protects.
Eimonte, Paulauskas, Daniusevičiūtė et al. Int J Hyperthermia. 2021;38(1):696-707. DOI: 10.1080/02656736.2021.1915504Who: Cain and colleagues analysed eleven randomised studies with a total of 3177 participants on cold water immersion in healthy adults in 2025 in PLOS ONE. Water temperatures ranged between 7 and 15 degrees, durations between 30 seconds and two hours.
What they found: Inflammatory markers rose significantly immediately after the cold, with a standardised mean difference of 1.03 and a confidence interval of 0.37 to 1.68, and more strongly still one hour later. For immune function no significant effect was found, neither immediately nor after one hour. Stress was significantly reduced twelve hours after the exposure, at all other measurement points it was not. Sleep quality and quality of life improved, mood did not.
What that means for you: This is the most honest overall view we currently have. Cold produces more inflammation in the short term, not less. The favourable effects appear with a delay. That is exactly the pattern one would expect with hormesis.
Cain, Brinsley, Bennett, Nelson, Maher, Singh. PLoS One. 2025;20(1):e0317615. DOI: 10.1371/journal.pone.0317615- Cold shock response and catecholamine rise. Physiologically well described and reproducibly measurable.
- Habituation. The reaction to cold attenuates markedly after a few sessions and stays dampened for months.
- Brown adipose tissue. Activatable under mild cold in almost all adults, and increasable over weeks.
- Fewer days of sickness absence. In a large randomised trial 29 percent fewer, although self-reported.
- Stress reduction with a delay. In the meta-analysis significant twelve hours after the exposure.
- Sleep and quality of life. Improved in the meta-analysis, although on a narrow data base.
- Fewer infections. The number of sick days did not differ significantly in the large trial.
- Better immune function right after the cold. No significant effect in the meta-analysis.
- Clinical meaning of the laboratory shifts. Described by the authors themselves as unresolved.
- Are winter swimmers healthier? Unclear, because self-selection distorts the observational data.
- An optimal dose. Temperatures from 7 to 15 degrees, durations from 30 seconds to two hours, no derivable recommendation.
- Cold as a therapy in autoimmune disease. Discussed mechanistically, not established clinically.
And now you know why, on this topic, I distinguish between fewer sickness absences and less illness.
The well known breathing studies: why the breathing is not the cold
There is one study that practically everyone who looks into this topic knows. It is good, it is fascinating, and it is almost always reported in abbreviated form.
Who: Kox, Pickkers and colleagues from Nijmegen published a study in 24 healthy volunteers in 2014 in the Proceedings of the National Academy of Sciences. Twelve went through a ten-day training, twelve did not.
What they found: After intravenous administration of bacterial endotoxin, the trained group showed markedly increased plasma epinephrine levels. The anti-inflammatory interleukin 10 rose faster and higher and correlated strongly with the preceding epinephrine levels. The pro-inflammatory messengers TNF-alpha, interleukin 6 and interleukin 8 were lower. Flu-like symptoms occurred less often.
What that means for you: That is a remarkable finding. People can influence a reaction that was considered involuntary. It is just that the training consisted of three components at the same time: meditation, a breathing technique with cyclic hyperventilation and breath holding, and cold exposure. Which component carries the effect, this study cannot say.
Kox, van Eijk, Zwaag et al. Proc Natl Acad Sci U S A. 2014;111(20):7379-7384. DOI: 10.1073/pnas.1322174111That very question was answered by the same research group eight years later. And the answer surprised me the first time I read it.
Who: Zwaag, Naaktgeboren, Pickkers and Kox published a paper in 2022 in Psychosomatic Medicine that took the components apart. 48 healthy men were randomised to cold training alone, the breathing exercise alone, the combination or no training. Afterwards all of them received endotoxin.
What they found: Cold training alone did not change the endotoxin-induced inflammatory response relevantly, with p equal to 0.77. The breathing exercise, by contrast, significantly increased the anti-inflammatory and lowered the pro-inflammatory messengers, with p equal to 0.002. Cold training additionally amplified the immune-modulating effects of the breathing exercise, with p equal to 0.02.
What that means for you: The immunologically active part appears to be the breathing, not the cold. Cold could act as an amplifier. That is something entirely different from the statement that cold strengthens the immune system.
Zwaag, Naaktgeboren, van Herwaarden, Pickkers, Kox. Psychosom Med. 2022;84(4):457-467. DOI: 10.1097/PSY.0000000000001065When someone says that science has proven that cold influences the immune system, they almost always mean the 2014 study. And they overlook the 2022 study, in which the same group separated the components.
This is not a criticism of the method, by the way. It is a good example of how science works: first you show that something is possible. Then you ask which part it was.
And now you know why, in the Wim Hof debate, I ask about the breathing first.
When cold becomes counterproductive
I have been taking ice cold showers every morning for months and still keep getting ill. What am I doing wrong?
I hear this question regularly, and it almost always comes with an undertone of disappointment. My first counter-question is rarely the expected one. I do not ask about the water temperature.
Very often a pattern then appears: high continuous load, short sleep, plus an additional stress stimulus every morning. From my point of view, cold in such a situation is not a training impulse. It is another item on an account that does not balance anyway.
In these conversations, cold training is almost always just one component among many. Usually the one carrying the most hope, and the one carrying the least weight as long as sleep and recovery are not in order.
I write this without any claim of causality. It is a clinical observation, not a study result. But it is the observation that has most often changed something.
For me, this is where the core of the KPNI idea sits on this topic.
A hormetic stimulus can only achieve something if recovery follows it. Mattson already puts it that way: the adaptation does not arise in the stimulus, but in the response to it. And that response costs resources.
If your nervous system is running in permanent sympathetic mode anyway, your sleep is too short and your immune system is currently working on something, then exactly that capacity is missing. The stimulus arrives. The adaptation does not.
Situations in which I would rather put cold on hold
- During an acute infection. The immune system is already working. The meta-analysis shows a rise in inflammatory markers right after cold, not a calming.
- In deep exhaustion. When everyday demands are already too much, an additional stress stimulus is not a good idea.
- With chronic stress and no recovery phases. Without regeneration the stimulus turns into a load. That is the core statement of the hormesis concept, simply read the other way round.
- With lack of sleep over a longer period. Adaptation processes run predominantly in phases of rest. If the rest is missing, the adaptation is missing.
- Directly after strength training. More on that in a moment, this point is surprisingly well studied.
The side note for everyone who trains
This point deserves a paragraph of its own, because it is regularly done wrong in gyms.
Who: Roberts and colleagues studied 21 physically active men in 2015 in the Journal of Physiology over twelve weeks of strength training, twice a week. One group did ten minutes of cold water immersion after each session, the other active recovery.
What they found: Strength and muscle mass increased more in the group with active recovery. There, isokinetic work rose by 19 percent, the cross-sectional area of type II fibres by 17 percent and the number of nuclei per fibre by 26 percent. In the cold group these gains did not appear. In a second part of the experiment, the activation of satellite cells and of the growth-regulating signalling pathways was also lower after cold.
What that means for you: The muscle needs the inflammatory signal after training as a building instruction. Anyone who cools it immediately may dampen exactly the stimulus they trained for. The authors write explicitly that the routine use of cold water immersion after training should be reconsidered.
Roberts, Raastad, Markworth et al. J Physiol. 2015;593(18):4285-4301. DOI: 10.1113/JP270570Two good stimuli at the wrong time can cancel each other out. That is not esoterics, that is timing.
Shukri Jarmoukli, ViveCura BerlinAnd now you know why I always ask about recovery before asking about temperature.
Limits and dangers: the section I consider the most important
Up to here it was a discussion about benefit. Now it gets more serious.
Cold water is not a wellness product. It is a physiological load that can cost people their lives. That sounds harsh, but it is the sober state of the literature.
Who: Tipton, Collier, Massey, Corbett and Harper presented the first broad overview of cold water immersion as a danger and as a treatment in 2017 in Experimental Physiology.
What they state: Cold water immersion is described as a precursor to drowning, cardiac arrest and hypothermia, while at the same time its therapeutic applications are assessed. The authors rate the evidence base of the individual claims very differently. For some effects a plausible rationale exists, for others the data remain at the level of anecdotal speculation.
What that means for you: The very same review that sorts the benefits names the danger in the same breath. Every serious recommendation on this topic should do that too.
Tipton, Collier, Massey, Corbett, Harper. Exp Physiol. 2017;102(11):1335-1355. DOI: 10.1113/EP086283Who: Shattock and Tipton described a mechanism in 2012 in the Journal of Physiology that they call autonomic conflict.
What they describe: Submersion in cold water can produce a high rate of cardiac arrhythmias even in healthy volunteers. The reason: the cold shock response and the diving reflex are activated at the same time. One drives the heart rate up via the sympathetic branch, the other drives it down via the parasympathetic branch. Both branches of the autonomic nervous system pull in opposite directions simultaneously. The authors suspect that some deaths previously attributed to drowning or hypothermia actually go back to this.
What that means for you: Submerging the head and holding your breath in cold water is precisely the riskiest part. Not the cold itself.
Shattock, Tipton. J Physiol. 2012;590(14):3219-3230. DOI: 10.1113/jphysiol.2012.229864Please have this medically assessed beforehand
- Heart disease and arrhythmias. Cold shock and the diving reflex together can produce arrhythmias, even in people healthy until now.
- High blood pressure. Cold raises blood pressure and heart rate acutely. With poorly controlled hypertension that is risky.
- Pregnancy. The data are practically non-existent. Without data I advise restraint.
- Raynaud phenomenon. Cold is the classic trigger here for the vascular spasms in fingers and toes.
- Cold urticaria. Weals or swelling after cold exposure, usually on rewarming. Anaphylaxis is possible, especially with whole-body exposure.
- Underactive thyroid. Heat production is reduced here anyway. That makes counter-regulation more difficult.
- Underweight and eating disorders. Little insulation, little muscle mass, often circulatory instability. Cooling down happens considerably faster.
- Non-swimmers. In open water, cold exposure without secure swimming ability is not an option.
Who: Maltseva, Maurer and colleagues summarised the state of knowledge on cold urticaria in 2021 in Allergy.
What they describe: Cold urticaria is a common form of chronic inducible urticaria. After cold exposure, weals, angioedema or both appear. The weals usually form on rewarming and subside within an hour. Anaphylaxis can occur. The diagnosis rests on the history and a cold stimulation test. Treatment includes cold avoidance, non-sedating antihistamines and, in off-label use, omalizumab.
What that means for you: If your skin itches, turns red or forms weals after cold, that is not a sign of hardening. It is a finding that belongs in a medical assessment before you step into cold water.
Maltseva, Borzova, Fomina et al. Allergy. 2021;76(4):1077-1094. DOI: 10.1111/all.14674And now you know why, with me, every conversation about cold starts with the question about heart, skin and thyroid.
How I place this: principles instead of a protocol
I deliberately do not hand out a table with seconds and degrees here. There are two reasons for that.
First, the data do not support it. The reviewed studies work with temperatures between 7 and 15 degrees and durations between 30 seconds and two hours. No optimal dose can be derived from that. In the large cold shower trial, 30 seconds performed similarly to 90.
Second, the appropriate dose depends on your history. A protocol from the internet knows neither your heart nor your sleep nor your current load.
What I offer instead are three principles.
Principle 1: short and regular beats long and harsh
- Habituation research supports this directly. Six sessions of three minutes each were enough to dampen the cold shock response markedly.
- The stimulus does not have to be maximal. The habituation was achieved at 15 degrees and afterwards also carried over to 10 degrees.
- With hormesis, more cold is not more effect. The curve is biphasic, not rising.
- For most people the shower is enough. It is controllable, can be stopped at any moment, and carries no risk of drowning.
Principle 2: the stimulus is only half the account
- The adaptation arises afterwards. In the hours of rest, not in the seconds of cold.
- Sleep first. If sleep is not in order, an additional stress stimulus is the wrong order of things.
- Exhaustion is a stop signal. Not a reason to get tougher, but a reason to look at the cause first.
- Pause during an infection. The system currently has a different task.
Principle 3: safety before effect
- Never alone in open water. Always a second person, a known exit point, no alcohol.
- No breath holding techniques in the water. Loss of consciousness can occur without warning.
- With pre-existing conditions, have it medically assessed beforehand. Heart, blood pressure, thyroid, skin, pregnancy.
- Listen to your own body. Persistent shivering, numbness, confusion or palpitations are signs to stop, not a training threshold.
Cold is a good tool for practising that discomfort does not mean danger. This psychological part is almost always underestimated in the discussion.
Many people report that after a cold shower they go into the day not healthier, but more capable of acting. That is an effect I take seriously. I just call it by its name then, instead of selling it as an immune effect.
If you want not only to read about this but to have your susceptibility to infections properly assessed, the next step is a medical examination.
And now you know why I prefer principles to protocols.
Cold in context
Cold does not stand on its own. It reaches into the nervous system, into metabolism and into recovery. In clinical practice we therefore never look at a single stimulus alone.
Cold
Brief stress stimulus, hormesis, adaptation instead of permanent load
this articleSleep
The place where the adaptation after the stimulus actually happens
Stress and nervous system
Sympathetic tone, catecholamines and the question of how much load is already there
Immune system
Susceptibility to infections usually has several causes, rarely just one
Frequently asked questions about cold, hormesis and the immune system
Does ice bathing strengthen the immune system?
The honest answer is: perhaps a little, but not in the way it is usually sold. What is established is that cold sets off a strong stress reaction with a rise in norepinephrine, and that the body habituates measurably to repeated cold. A large Dutch trial with more than 3000 participants found 29 percent fewer self-reported days of sickness absence under daily cold showering. The number of sick days itself, however, did not differ significantly. A meta-analysis from 2025 found no measurable improvement in immune parameters right after the cold, but rather a short-term rise in inflammatory markers. Cold can be a training stimulus. It is not immune protection.
What exactly does hormesis mean?
Hormesis describes a biphasic dose response relationship. A small amount of a stimulus may prompt an adaptive, favourable response, while a large amount of the same stimulus can do harm. In biology and medicine the term refers to the adaptive reaction of cells and organisms to moderate, usually time-limited stress. Well known examples are physical training, dietary restriction and ischemic preconditioning. Calabrese and Mattson describe the typical stimulatory range as roughly 30 to 60 percent above the control value. The decisive word is moderate. Hormesis is not a licence for more, it is an argument for the appropriate dose.
What happens in the first seconds in cold water?
This first phase is called the cold shock response. The cold receptors of the skin fire abruptly. What follows is an involuntary gasp, a barely controllable hyperventilation and a rise in heart rate. In one study in volunteers, the respiratory rate in the first 30 seconds reached up to 47 breaths per minute and the heart rate 128 beats per minute. This phase is precisely the most dangerous part, because hyperventilation under water and above the water can throw the breathing rhythm out of control. It typically lasts the first two to three minutes.
What did the Dutch cold shower trial show?
Buijze and colleagues randomised 3018 adults between 18 and 65 years in 2015 to a warm-to-cold shower of 30, 60 or 90 seconds or to a control group, over 30 consecutive days. 79 percent completed the protocol. Result: 29 percent fewer self-reported days of sickness absence, with an incidence rate ratio of 0.71 and p equal to 0.003. For the number of sick days itself there was no significant group effect. There were no serious adverse events. Important for placing this: the trial was not blinded, the endpoints were self-reported, and the duration of the shower made no discernible difference.
Is the Wim Hof method scientifically established?
There is genuine human research on it, and it is remarkable. Kox and colleagues showed in 2014 that trained participants, after being given bacterial endotoxin, had markedly higher epinephrine levels, more anti-inflammatory interleukin 10 and less TNF-alpha, interleukin 6 and interleukin 8 than untrained participants. The training, however, consisted of three components at the same time: meditation, a breathing technique and cold. A follow-up study by the same group from 2022 separated the components. There, cold training alone did not change the inflammatory response relevantly. The breathing exercise did. The combination was the strongest. Based on these data, the immune-modulating core appears to lie in the breathing rather than in the cold.
How long does it take for the body to get used to cold?
Surprisingly little time. In one study, six short immersions of three minutes each were enough to dampen the cold shock response markedly. In the first 30 seconds the respiratory rate fell from 47 to 24 breaths per minute, the minute ventilation from 68 to 30 litres per minute and the heart rate from 128 to 109 beats per minute. In a follow-up study this habituation was still present after seven months, and after 14 months only the dampened heart rate remained. Another interesting point: the habituation was achieved in water at 15 degrees and afterwards also carried over to 10 degrees. So the stimulus does not have to be maximal for the body to respond.
Can I take cold showers or ice baths while I have an infection?
With an acute infection I advise against it. A hormetic stimulus presupposes that the organism currently has capacity for an adaptation. During an ongoing infection the immune system is already working on a task, and a meta-analysis from 2025 showed a rise in inflammatory markers immediately after cold exposure rather than a calming effect. Added to this is the strong sympathetic activation with a rise in blood pressure and heart rate, which is inappropriate with fever or circulatory weakness. The same applies to phases of deep exhaustion and to chronic stress without recovery phases. Such a stimulus then does not become an adaptation, it becomes additional load.
For whom is ice bathing dangerous?
Cold water exposure is not a harmless wellness application. Cold water can trigger cardiac arrhythmias, even in healthy volunteers. Shattock and Tipton describe the concept of autonomic conflict for this: the cold shock response and the diving reflex activate both branches of the autonomic nervous system at the same time. Caution is warranted with known heart disease, arrhythmias, poorly controlled high blood pressure, in pregnancy, with Raynaud phenomenon, with cold urticaria, with an underactive thyroid, with underweight and eating disorders, in non-swimmers and after alcohol. In open water the risk of drowning is added. With pre-existing conditions, a medical assessment belongs before the first attempt.
Does cold after strength training harm muscle growth?
There are serious indications for this. In an Australian study, 21 physically active men trained with weights twice a week for twelve weeks. One group did ten minutes of cold water immersion afterwards, the other active recovery. Strength and muscle mass increased more in the group with active recovery. Cross-sectional area of the type II fibres and the number of nuclei per fibre rose significantly only there. The explanation is mechanistically coherent: the muscle needs the inflammatory signal after training as an adaptive stimulus. Anyone who wants to use both cold and strength training would sensibly separate them in time.
Cold shower or ice bath: which makes more sense?
For the vast majority of people the cold shower is the more sensible choice. It is the only form of cold exposure for which a large randomised human trial exists. It is controllable, can be stopped at any moment, and there is no risk of drowning. With an ice bath in open water, cold shock, incapacitation through cooling and the unpredictability of natural waters are added. From habituation research we also know that the stimulus does not have to be maximal to prompt an adaptation. With hormesis, more cold is not automatically more effect.
How cold and how long should a cold application be?
I deliberately do not hand out a universal protocol, for two reasons. First, the study data are too thin for that: the available studies use temperatures between 7 and 15 degrees and durations between 30 seconds and two hours, without an optimal dose being derivable from them. In the large cold shower trial it made no discernible difference whether people showered for 30, 60 or 90 seconds. Second, the appropriate dose depends on your history, your cardiovascular status and your current load. The principles matter more than the numbers: short rather than long, regular rather than harsh, recovery afterwards, never alone in open water, and with pre-existing conditions have it medically assessed beforehand.
Scientific sources
- Mattson MP. Hormesis defined. Ageing Res Rev. 2008;7(1):1-7. DOI: 10.1016/j.arr.2007.08.007 [Review]
- Calabrese EJ, Mattson MP. How does hormesis impact biology, toxicology, and medicine? NPJ Aging Mech Dis. 2017;3:13. DOI: 10.1038/s41514-017-0013-z [Mechanism review]
- Šrámek P, Šimečková M, Janský L, Šavlíková J, Vybíral S. Human physiological responses to immersion into water of different temperatures. Eur J Appl Physiol. 2000;81(5):436-442. DOI: 10.1007/s004210050065 [In vivo, human]
- Tipton MJ, Golden FS, Higenbottam C, Mekjavic IB, Eglin CM. Temperature dependence of habituation of the initial responses to cold-water immersion. Eur J Appl Physiol Occup Physiol. 1998;78(3):253-257. DOI: 10.1007/s004210050416 [In vivo, human, n=13]
- Tipton MJ, Mekjavic IB, Eglin CM. Permanence of the habituation of the initial responses to cold-water immersion in humans. Eur J Appl Physiol. 2000;83(1):17-21. DOI: 10.1007/s004210000255 [In vivo, human, n=12]
- van Marken Lichtenbelt WD, Vanhommerig JW, Smulders NM, Drossaerts JM, Kemerink GJ, Bouvy ND, Schrauwen P, Teule GJ. Cold-activated brown adipose tissue in healthy men. N Engl J Med. 2009;360(15):1500-1508. DOI: 10.1056/NEJMoa0808718 [In vivo, human, n=24]
- Yoneshiro T, Aita S, Matsushita M, Kayahara T, Kameya T, Kawai Y, Iwanaga T, Saito M. Recruited brown adipose tissue as an antiobesity agent in humans. J Clin Invest. 2013;123(8):3404-3408. DOI: 10.1172/JCI67803 [RCT, crossover, human]
- Buijze GA, Sierevelt IN, van der Heijden BC, Dijkgraaf MG, Frings-Dresen MH. The Effect of Cold Showering on Health and Work: A Randomized Controlled Trial. PLoS One. 2016;11(9):e0161749. DOI: 10.1371/journal.pone.0161749 [RCT, n=3018]
- Kox M, van Eijk LT, Zwaag J, van den Wildenberg J, Sweep FC, van der Hoeven JG, Pickkers P. Voluntary activation of the sympathetic nervous system and attenuation of the innate immune response in humans. Proc Natl Acad Sci U S A. 2014;111(20):7379-7384. DOI: 10.1073/pnas.1322174111 [RCT, n=24]
- Zwaag J, Naaktgeboren R, van Herwaarden AE, Pickkers P, Kox M. The Effects of Cold Exposure Training and a Breathing Exercise on the Inflammatory Response in Humans: A Pilot Study. Psychosom Med. 2022;84(4):457-467. DOI: 10.1097/PSY.0000000000001065 [RCT, n=48]
- Janský L, Pospíšilová D, Honzová S, Uličný B, Šrámek P, Zeman V, Kamínková J. Immune system of cold-exposed and cold-adapted humans. Eur J Appl Physiol Occup Physiol. 1996;72(5-6):445-450. DOI: 10.1007/BF00242274 [In vivo, human]
- Eimonte M, Paulauskas H, Daniusevičiūtė L, Eimantas N, Vitkauskienė A, Dauksaite G, Solianik R, Brazaitis M. Residual effects of short-term whole-body cold-water immersion on the cytokine profile, white blood cell count, and blood markers of stress. Int J Hyperthermia. 2021;38(1):696-707. DOI: 10.1080/02656736.2021.1915504 [In vivo, human]
- Cain T, Brinsley J, Bennett H, Nelson M, Maher C, Singh B. Effects of cold-water immersion on health and wellbeing: A systematic review and meta-analysis. PLoS One. 2025;20(1):e0317615. DOI: 10.1371/journal.pone.0317615 [Meta-analysis, k=11, n=3,177]
- Esperland D, de Weerd L, Mercer JB. Health effects of voluntary exposure to cold water: a continuing subject of debate. Int J Circumpolar Health. 2022;81(1):2111789. DOI: 10.1080/22423982.2022.2111789 [Systematic review, k=104]
- Roberts LA, Raastad T, Markworth JF, Figueiredo VC, Egner IM, Shield A, Cameron-Smith D, Coombes JS, Peake JM. Post-exercise cold water immersion attenuates acute anabolic signalling and long-term adaptations in muscle to strength training. J Physiol. 2015;593(18):4285-4301. DOI: 10.1113/JP270570 [RCT, n=21]
- Tipton MJ, Collier N, Massey H, Corbett J, Harper M. Cold water immersion: kill or cure? Exp Physiol. 2017;102(11):1335-1355. DOI: 10.1113/EP086283 [Review]
- Shattock MJ, Tipton MJ. Autonomic conflict: a different way to die during cold water immersion? J Physiol. 2012;590(14):3219-3230. DOI: 10.1113/jphysiol.2012.229864 [Mechanism review]
- Maltseva N, Borzova E, Fomina D, Bizjak M, Terhorst-Molawi D, Košnik M, Kulthanan K, Meshkova R, Thomsen SF, Maurer M. Cold urticaria: what we know and what we do not know. Allergy. 2021;76(4):1077-1094. DOI: 10.1111/all.14674 [Review]