Cold Plunge and Cortisol: What the Research Shows
Key insights
- In the best-controlled laboratory comparison available, one hour of head-out immersion in 14 degree Celsius water raised plasma noradrenaline by 530% and dopamine by 250%, while cortisol did not rise at all and if anything drifted slightly downwards 1.
- A 2025 crossover trial of five-minute ice baths at 8 to 12 degrees Celsius found noradrenaline roughly doubling within five minutes in both morning and evening sessions, while cortisol simply followed its normal daily rhythm and showed no response to the plunge itself 2.
- Across 12 weeks of thrice-weekly ice-water swimming or whole-body cryotherapy, the noradrenaline response held steady while ACTH and cortisol at 35 minutes fell significantly below week-one values, which the authors read as habituation rather than stimulation of the pituitary-adrenal axis 3.
- A 2025 meta-analysis of 11 randomised trials found no change in self-reported stress immediately, one hour, 24 hours or 48 hours after immersion, but a large reduction at 12 hours (SMD -1.00), alongside a measurable rise in inflammatory markers during the first hour 4.
- Even at the extreme end of the dose range, a single immersion in water below 4 degrees Celsius in minus 15 degree air produced no significant change in blood cortisol, though testosterone fell sharply and several muscle and liver enzymes rose within non-pathological limits 5.
Few claims travel further with less scrutiny than the idea that cold plunging lowers cortisol. It appears in tub marketing, in podcast summaries and in the internal logic most people use to justify the habit: cold is stress, the body adapts to stress, therefore the stress hormone must come down. It is a tidy story, and it is the one part of the cold-exposure literature that is almost entirely backwards.
What the human data actually show is that cortisol is largely a bystander during cold water immersion. The hormone that moves, and moves enormously, is noradrenaline. Cortisol either stays flat or drifts down with its own daily rhythm, and this holds across water temperatures from 32 degrees Celsius down to below 4, across exposures from 20 seconds to a full hour, and in men and women alike.
That is a more interesting finding than the marketing version, because it changes what a plunge is for. What follows is the endocrinology, the human studies that carry most of the weight, what changes after weeks of repeated exposure, and what the trial evidence says about stress as people actually experience it.
Two stress systems, not one
The body has two separate ways of answering a threat, and they run on different clocks. The sympatho-adrenal system fires within seconds, releasing noradrenaline and adrenaline that raise heart rate, constrict peripheral vessels and mobilise fuel. The hypothalamic-pituitary-adrenal axis is slower: the hypothalamus releases corticotropin-releasing hormone, the pituitary releases ACTH, and the adrenal cortex responds by secreting cortisol, typically peaking 15 to 30 minutes after a stressor begins.
Cortisol also has a strong circadian rhythm of its own, rising steeply on waking and declining across the day. Any measurement of cortisol around a plunge is therefore measuring two things at once: whatever the cold did, and where the person happens to sit on that daily curve. This is the single most common source of confusion in the popular discussion, and it is why time-of-day-controlled studies matter so much here.
The reasonable prediction, before looking at the data, is that a stimulus as severe as ice water would recruit both systems. It recruits one.
What actually happens to cortisol during a plunge
The cleanest experiment on this question is more than two decades old and still has not been bettered for design. Ten young men underwent one-hour head-out immersions in water at 32, 20 and 14 degrees Celsius, in random order, one week apart, all in the morning, with resting control measurements in air. At 14 degrees, rectal temperature fell by 1.7 degrees and metabolic rate rose by about 350%. Plasma noradrenaline went from 1.17 to 6.20 pmol per ml, an increase of 530%, and dopamine rose 250%. Adrenaline did not change. Cortisol tended to decrease at every water temperature tested, and at no point differed significantly from the resting controls. The authors' conclusion was blunt: water immersion and cold did not activate the hypothalamus-pituitary-corticoadrenal axis in the way stress conditions typically do 1.
An hour is a laboratory exposure rather than a plunge protocol, so the obvious question is whether a short, sharp immersion behaves differently. A 2025 crossover study answered it directly. Six women and six men took five-minute ice baths at 8 to 12 degrees Celsius, once in the morning and once in the evening on separate days, with the preceding 24 hours of food and exercise standardised. Noradrenaline rose 127% in the morning and 144% in the evening within five minutes of getting out. Cortisol differed markedly between sessions, but along the axis of the clock rather than the cold: 179 pg per ml before the morning bath against 91 before the evening one, with the same roughly two-fold gap at five and 30 minutes afterwards. The study's own headline conclusion was that the hormonal response to an ice bath is essentially the same morning or evening, and there was no difference between women and men 2.
If the effect were simply a matter of insufficient dose, pushing the temperature lower should reveal it. In 2025 a group immersed 13 young men in water below 4 degrees Celsius at an air temperature of minus 15. That exposure moved a great deal of blood chemistry: creatine kinase, CK-MB, lactate dehydrogenase, bilirubin and the transaminases all rose, and testosterone fell sharply, all within non-pathological limits. Cortisol did not change significantly, and drifted slightly down 5. At the far end of the intensity range, the HPA axis still declines to participate.
What changes after weeks of doing it
The strongest version of the popular claim is not about a single session but about adaptation over time, and there is one study that speaks to it properly. Twenty healthy women were exposed three times a week for 12 weeks, half to winter swimming in water at 0 to 2 degrees Celsius for 20 seconds, half to whole-body cryotherapy at minus 110 degrees for two minutes. Blood was drawn in weeks 1, 2, 4, 8 and 12, both on non-exposure days and on exposure days before, five minutes after and 35 minutes after the cold 3.
Two things separated over those 12 weeks. The noradrenaline response was sustained: it appeared with every exposure, at similar magnitude in both modalities, and it did not fade with repetition. ACTH and cortisol at the 35-minute timepoint, by contrast, were significantly lower in weeks 4 through 12 than in week 1. The authors attributed this to habituation and concluded that neither winter swimming nor cryotherapy stimulated the pituitary-adrenal cortex axis.
This is the closest thing in the literature to evidence that regular cold exposure lowers cortisol, and it is worth being precise about what it is. It is a fall in the corticoid response to a repeated cold stimulus within a cold-exposed group. It is not a demonstration of lower resting cortisol across the day, and it is not evidence that plunging fixes chronically elevated cortisol from work, sleep debt or illness. What the data describe is a nervous system that keeps its catecholamine response intact while learning to stop treating the cold as an emergency.
What the randomised trials say about stress itself
Hormone concentrations are a proxy. The more practical question is whether people feel less stressed, and a 2025 systematic review and meta-analysis pooled 11 randomised trials covering 3,177 participants, all using water at 15 degrees Celsius or below for at least 30 seconds. On stress the pattern was strikingly time-dependent: no significant change immediately after immersion, at one hour, at 24 hours or at 48 hours, but a large and significant reduction at 12 hours (SMD -1.00, 95% CI -1.40 to -0.61), rated Grade B certainty 4.
The same review found that inflammation rose significantly immediately after immersion (SMD 1.03) and remained elevated an hour later (SMD 1.26), while measures of immunity did not change at either timepoint. Cold water immersion, on this evidence, is not a sedative. It is an acute physiological insult with a delayed calming effect, and the delay is long enough that most people never connect the two.
These were self-reported stress scales rather than cortisol assays, so the two literatures are measuring different things. They point in the same direction. Nothing dramatic happens to the stress axis in the moment; something modest and possibly useful happens later.
Realistic expectations
If the reason you plunge is to lower cortisol, the evidence does not support the goal or the mechanism. Cortisol is not what cold water moves. If you are tracking it on a wearable or a saliva panel and expecting the number to fall, you are likely to be measuring your own circadian rhythm and the time you happened to test.
It is also worth saying that cortisol is not a villain. The morning rise is functional, it mobilises fuel and helps you get out of bed, and suppressing it is not a health objective in itself. The thing people are actually feeling after a plunge is a large, sustained noradrenaline release, the same mechanism behind the sharp jump in metabolic rate during cold immersion, combined over time with the practised experience of staying composed inside a controlled stressor. Those are defensible reasons to do it. Cortisol reduction is not.
The 12-hour finding is the most practically interesting and the least discussed. If the stress benefit lands roughly half a day later, a morning plunge would be expected to pay off in the evening. There is no evidence that plunging in the morning produces a better hormonal response than plunging at night; the crossover data found the two essentially identical.
Protocol and practical guidance
The trials that measured hormones used water between 8 and 14 degrees Celsius, and the meta-analysis threshold was 15 degrees or below. That range is the evidence base. Colder is not better here, and the sub-4-degree study is a reminder that pushing the extreme mostly buys you higher creatine kinase rather than a bigger endocrine signal.
On duration, the full catecholamine response appears within the first few minutes; the five-minute ice baths produced it, and the hour-long immersions were laboratory instruments rather than recommendations. Two to five minutes at 10 to 15 degrees, two or three times a week, is consistent with everything above. Enter under control rather than jumping in, keep breathing slow through the first 30 seconds of the cold shock response, and never plunge alone.
Cold immersion raises blood pressure and heart rate acutely. If you have cardiovascular disease, an arrhythmia, uncontrolled hypertension, or are pregnant, speak to a clinician before starting, and do not treat the discomfort as the objective.
The Contrast Market Perspective
The through-line in this research is narrowness of dose. Every study above worked because the water sat in a defined band, held there session after session, and was verified rather than estimated. That is what separates an adaptation from an anecdote, and it is the one variable a home setup most often gets wrong: a tub that drifts several degrees between sessions is running a different experiment each time. If you are specifying a cold plunge or contrast setup and want it built around stable, measurable temperature rather than guesswork, Schedule a consultation and we will help you get the details right.
References
Every claim above is linked to peer-reviewed research, listed in full below.
Footnotes
- Šrámek P, Šimečková M, Janský L, Šavlíková J, Vybíral S (2000). Human physiological responses to immersion into water of different temperatures. European Journal of Applied Physiology. PubMed ↩︎
- Braunsperger A, Bauer M, Ben Brahim C, et al. (2025). Effects of time-of-day on the noradrenaline, adrenaline, cortisol and blood lipidome response to an ice bath. Scientific Reports. PubMed ↩︎
- Leppäluoto J, Westerlund T, Huttunen P, Oksa J, Smolander J, Dugué B, Mikkelsson M (2008). Effects of long-term whole-body cold exposures on plasma concentrations of ACTH, beta-endorphin, cortisol, catecholamines and cytokines in healthy females. Scandinavian Journal of Clinical and Laboratory Investigation. PubMed ↩︎
- Cain T, Brinsley J, Bennett H, Nelson M, Maher C, Singh B (2025). Effects of cold-water immersion on health and wellbeing: a systematic review and meta-analysis. PLOS ONE. PubMed ↩︎
- Teległów A, Cicha I (2025). Single immersion in cold water below 4 °C: a health hazard in young healthy men? PLOS ONE. PubMed ↩︎