If hormesis as a concept is new to you, start with the framework. This article is about what that framework means in practice when you step into cold water
The first cold plunge makes your cells worse.
That is not a metaphor. When researchers at the University of Ottawa’s Human and Environmental Physiology Research Unit immersed healthy adults in 14°C water and examined their immune cells afterwards, they found something that should unsettle anyone who has ever stepped into an ice bath expecting immediate benefit: autophagy, the body’s cellular clean-up system, was impaired. Apoptosis signals, markers of cell death, were elevated. The stress response looked like damage, not adaptation.
Seven days later, using the same water, the same temperature, the same people, the picture had reversed. Autophagy was enhanced. Damage markers had dropped. Cold shock proteins and heat shock proteins had spiked early in the week, then settled into a steady, functional baseline. “We were amazed to see how quickly the body adapted,” said Kelli King, a PhD researcher at the University of Ottawa and the study’s lead author.
Hormesis explains this reversal: why measured stress first harms, then strengthens. Understanding it changes how you think about cold exposure, dose, consistency, and why your worst session might be the most important one.
What hormesis actually means
Hormesis describes a biphasic dose-response: a moderate dose of stress triggers beneficial adaptation, while a high dose causes harm. The principle, mapped as an inverted U-curve, is explained in full here, what matters for cold exposure is how that curve applies to specific temperatures, durations, and frequencies.
What happens in your cells during cold immersion
When cold water hits skin, the body doesn’t begin recovering. It begins panicking. Peripheral vessels constrict, heart rate spikes, and norepinephrine floods the bloodstream, producing the alertness and euphoria most people associate with a cold plunge. This acute cascade is the alarm, not the adaptation. Adaptation begins afterwards, and over repeated sessions during immersion itself, through cellular programmes that each represent a distinct expression of the hormetic principle.
Autophagy, the cell’s internal recycling system, tags damaged proteins and dysfunctional organelles for removal and replacement. King and Kenny’s data showed cold immersion eventually enhances autophagic activity, but only after the initial sessions impair it. Repair crews have to be mobilised before they can work.
Cold shock proteins, particularly RBM3, are produced in response to drops in core body temperature and appear to protect neurons from degeneration, decreasing neuroinflammation and supporting synaptic function. RBM3 research remains early-stage but promising.
Heat shock proteins (HSP70 and HSP90) sound counterintuitive in a cold context, but the body produces them in response to any significant cellular stress. In King and Kenny’s data, they spiked during the first days, then levelled off as the body acclimated — molecular chaperones that stabilise other proteins under pressure.
Mitochondrial biogenesis, the creation of new mitochondria, is triggered when existing energy systems are stressed beyond comfortable capacity. Cold forces the body to generate heat, creating exactly that demand, and over time cells respond by building more and better mitochondria.
Anti-inflammatory adaptation is the most practically relevant shift for regular cold plungers. Acute cold exposure produces an initial inflammatory response; repeated exposure shifts the balance toward anti-inflammatory signalling, reducing chronic low-grade inflammation over weeks and months. Over time, the body learns to resolve inflammation more efficiently — taught by the very stressor that initially provoked it.
Each programme operates on its own timeline and responds to its own dose thresholds. None is the reason cold exposure works. All are expressions of the same underlying logic: controlled stress, repeated at a tolerable intensity, forces cells to build capacity they would not build in comfort.
From dysfunction to adaptation: what happens across a week
What makes the King and Kenny study most valuable isn’t any single biomarker. It is the arc: the shape of what happens between Day 1 and Day 7.
Initial dysfunction gave way, within seven days, to enhanced cellular function. What matters is the temporal structure: the adaptation happened between sessions, not during them. Cold was the stimulus; recovery was where the body rebuilt, stronger than before. “This enhancement allows cells to better manage stress,” said Glen Kenny, professor of physiology at the University of Ottawa and the study’s senior author, “and could have important implications for health and longevity.”
That arc maps onto lived experience in a way almost every cold plunge user will recognise. Day 1 is miserable: the shock grips your chest, breathing is hard to control, and the minutes after feel more like recovery from a crisis than a wellness practice. By Day 4 or 5, something shifts. The shock is still real, but the body handles it differently. By Day 7, the water has not changed. You have.
Put practically: the first session isn’t a preview of the practice. It is the worst version of it. People who try cold exposure once and decide it isn’t for them have experienced the stress without the payoff, because the payoff requires repetition. A guided first-timer protocol, whether at home or in a hospitality environment, should be designed around this reality. Starting at 10–12°C and progressing gradually over sessions mirrors the acclimation protocol itself and respects the Day 1 problem: a user’s willingness to come back for Day 2 is the entire game.
The dose question: how cold, how long, how often
If hormesis depends on dose, then the most important question is: what is the right dose of cold?
Honestly, the research points in several directions at once, depending on which outcome you care about.
Susanna Søberg, a metabolism researcher whose 2021 study on brown fat activation and cold-induced thermogenesis was published in Cell Reports Medicine, found that roughly 11 minutes of cold exposure per week, split across two to three sessions, was sufficient for measurable metabolic benefits. That threshold has become the most-cited dose recommendation in cold plunge culture, and for metabolic endpoints it is well-supported.
King and Kenny’s protocol differed: 14°C water, longer immersion times, seven consecutive days. Their target was cellular adaptation (autophagy, stress proteins, immune cell function), and the dose that produced those changes was both warmer and more frequent than a typical consumer protocol.
A 2025 review published by Healthspan adds a further complication. It argues that shivering-intensity exposure may be necessary for the deeper cellular repair pathways, and that brief, tolerable dips at very cold temperatures might produce the subjective effects of cold exposure (alertness, mood lift, norepinephrine) without triggering the hormetic programmes underneath. If this is correct, the popular two-minute plunge at 3°C may be producing a neurochemical experience rather than a cellular adaptation.
These findings aren’t contradictory — they measure different outcomes on different timelines with different protocols. But they make one thing clear: the optimal hormetic dose is not a single number. It depends on what you are trying to achieve, how often you practise, and how old you are.
An earlier study from King’s lab found that older males showed minimal autophagic activation from cold exposure, and that high-intensity cold increased apoptosis without the corresponding repair response. The hormetic window narrows with age. What counts as a beneficial dose for a thirty-year-old may be excessive for a sixty-five-year-old, making a starting temperature of 15°C more appropriate for older users than the 10°C that younger adults tolerate well.
Precise temperature control becomes critical here: not as a product feature, but as a dosing instrument. A plunge that can be set to 14°C for an acclimation-style protocol, adjusted to 15°C for an older user working within a narrower therapeutic range, or dropped to 7°C for an experienced user seeking shivering-intensity exposure is performing the same function as a calibrated training load. Your thermostat is the dose dial. It is the engineering that disappears once the ritual begins. Usage patterns in warmer climates reflect self-selected hormetic zones: around 70 per cent of users gravitate toward 10–12°C, while roughly 30 per cent push toward 3–4°C.
Alternating cold immersion with heat exposure, what practitioners call contrast therapy – compounds the hormetic stimulus, though the interaction between thermal stressors deserves its own treatment.
Where hormesis breaks down
One of the sharpest demonstrations that dose has a ceiling comes from Roberts et al. (2015), who found that cold water immersion after strength training substantially attenuated long-term gains in muscle mass and strength. Cold suppressed the inflammatory signalling muscles need to repair and grow. Two individually beneficial stressors, stacked together, cancelled each other out.
Roberts’s finding is not an argument against cold exposure. It is an argument for understanding that hormesis is context-dependent. A dose that enhances recovery on a rest day may blunt adaptation on a training day. One stressor can strengthen a system while undermining another if the timing is wrong. Longevity implications are real but still being established: animal models and cellular studies are compelling, while long-term human trials remain unavailable. Acknowledging this does not weaken the concept. It places it honestly within its current evidence base.
What hormesis changes about how you think about the practice
If you arrived here understanding that cold exposure produces specific benefits, more brown fat, better mood, reduced inflammation – you now understand something more structural. Those benefits are not separate mechanisms that happen to occur in cold water.
The practical consequence is simple and counterintuitive: consistency at a manageable dose matters significantly more than intensity at an extreme one. A plunge once a month does less than a moderate plunge three times a week, because the adaptation happens between sessions, not during them. Hormesis rewards frequency, not heroism.
Your thermostat matters more than your timer. And the plunge that felt worst, the one where your body hadn’t yet learned what the cold was for – was the one that started everything.
