How much time under cold temperature feels right for you?
Summary
Despite decades of research sponsored by armies all over the world, there is still no mathematical model of deliberate cold exposure that lends itself to measuring cold dose in healthy human beings. While Susanna Søeberg, PhD found that Danish winter swimmers averaged 11/min week, there still remains no optimal recommendation of time and temperature for cold plunge therapy.
The mechanisms of thermoregulation are the same in all people, but the extent of cold training determines the dosage. Physiologically, vasoconstriction, subcutaneous fat, metabolic flexibility, shivering and non-shivering thermogenesis all factor in cold tolerance and acclimation. However, the psychology of cold is also an important variable -- especially in cold water.
In this article, I proposes a new measure based on the principle of teime under temperature, for measuring your cold plunge dose and determining an optimum that works for you.
The Mathematics of Cold Hormesis
What is cold dose?
To avoid the dangers of accidental cold exposure, I always emphasize that your cold practice should be deliberate -- i.e., by design. The problem is that there is nothing in the scientific literature for measuring what we would call "dose" for cold plunge therapy.
Until now.
In this article, I propose the first scientific measure for determining how much time at what temperature constitutes a "cold dose" for a cold-acclimated individual. For those naive to cold exposure, the problem is much simpler. When a mature adult is just getting started in cold plunge, we assume they have little or no brown fat and thus near-zero non-shivering thermogenesis, weak vasoconstriction, minimal metabolic flexibility (i.e., switching from glucose to fat burning, and back again), and so we tell them:
When starting a new cold plunge practice, go cold enough to gasp and long enough to shiver.
This is an effective heuristic for people of all different body types and levels of subcutaneous (below the skin, not around the belly) fat, because when they feel the cold shock response that initiates the gasp reflex, we know that they've activated their sympathetic nervous system. When they begin muscle shivering, we know that they've signaled their bodies to recruit new brown fat.
However, after about two weeks of regular cold plunge therapy, a remarkable set of physiological changes take place in the healthy human body that strengthen thermoregulation and adapt the body to cold:
Brown fat forms around the base of the neck and at the top of the back between the scapula.
Smooth muscle tissues that control blood flow strengthen, and microvasculature in the fingers and toes improves.
Body fat composition changes, so that dangerous visceral (belly) and liver fat shrinks, while healthier subcutaneous fat increases.
Metabolic flexibility improves, meaning the body can switch from glucose to fat metabolism fast.
The anterior midcingulate cortex region of the brain grows, expanding what Andrew Huberman, PhD calls the "seat of the will to live."
Each one of these physiological changes improves the capacity of the body to thermoregulate by adapting better to cold. That's partly why thermofluid models of cold exposure developed by military scientists throughout the world have all failed to predict levels of cold tolerance that reliably predict impaired performance in cold weather soldiers -- because . The level of cold training, both physiological and psychological, is just as important.
What that means is that the same level of cold "exposure" in a person who is cold trained does not equal the same level of "dose," in comparison to someone who is naive to cold. That is, exposure is not the same as dose. Two people at different levels of cold training will obtain different results from the same time and temperature in the ice bath, just like two power lifters will obtain different benefits from the same amount of weight and repetition, depending on their state of training.
Cold Hormesis
When I was a doctoral student in environmental engineering, I took a course in toxicology that introduced me to one of most bedrock principles in public health called the dose-response curve. In most of our engineering classes, we learned that more is always better, to provide for a factor of safety. It's the same in business -- e.g., when it comes to profits, more is always better.
But in biology, that is not the case. What can be beneficial is small doses can be harmful in larger doses. In retrospect, this seems obvious. For example, you might enjoy one double cheeseburger, or even two, but most of us would become sick if we tried to force down three or four.
In biology, something can be good for you in small doses, and bad for you in larger doses. The relationship between dose and response is non-linear, such that too little of something results in a harmful insufficiency, but too much will turn a nutrient into a poison.
At the left edge of a typical dose-response curve, the dose (e.g., magnesium) is too low and the body suffers. As dose increases, the deficiency subsides and health improves, but at the right edge even essential nutrients become toxic when the dose is too high.
The same is true for deliberate cold exposure. Not enough cold, and your metabolism will suffer from ill-effects, including thyroid dysregulation and insulin resistance. Too much cold can lead to hypothermia.
Most people don't need a graduate class in toxicology to understand this, because it makes intuitive and experiential sense. That's probably why we often get the question, "How much ice bath is just the right amount?"
Measuring Cold Dose
Because none of the hundreds of scientific studies on deliberate cold exposure, cryotherapy, winter swimming, ice baths, or cold plunge therapy establish a standardized comparative measure for deliberate cold exposure dose, I decided to invent one for whole-body cold water immersion.
There are two important variables:
water temperature, and
time.
It stands to reason that the colder the water and the greater the time, the higher the dose.
But how should these variables be combined?
As a first approximation, heat transfers is proportional to temperature differences. When the water in he ice bath is colder than the temperature of your skin, then heat will transfer from your skin to the water. This model works well in metallurgy and steam engines, but the reality of the human body is more complex.
For example, with experience in the ice bath, your smooth muscle tissues adapt to improve vasoconstriction in the extremities and reduce heat transfer. Moreover, mitochondria in your brown fat burn glucose and fatty acids to generate heat via non-shivering thermogenesis, thereby keeping your core warm despite the heat that is extracted through the skin.
To get a measure of the effects of temperature on dose, we must first establish your thermal comfort line. This is the water temperature that feels neutral to you. Average skin temperature is cooler than core temperature -- about 80F -- so the average beginner won't feel cold in water that measures about 80F.
Let's call the difference between your thermal comfort line and water temperature your delta, which is the portion of the dose that is due to temperature effects.
To calculate your total dose, multiply your temperature delta by your time of whole body exposure. That means that someone with a thermal comfort line at 80F would have to spend twice as much time at 60F to get the same cold dose they would get at 40F.
More experienced cold plungers will have a lower thermal comfort line. From my experience with winter swimming, I find that anything above 60F barely registers for me as cold anymore. That is, my sense is that I could be in 60F water almost indefinitely, so for the sake of argument, I might take that as my thermal comfort line.
I keep my Forge at about 35F, so my typical temperature delta is 60F minus 35F, or +25F. A beginner with the thermal comfort line of 80F would experience a delta of 80F minus 35F, or +45F. That means it takes me 3.6 min to get the same dose that a beginner will get in 2 min.
Right now, this measure is hypothetical. To validate it, we'd have to gather several subjects, establish their thermal comfort line, and measure their skin temperature, core temperature, and other physiological makers to compare responses and see if they correspond to our estimate of dose. After that, we could establish a minimum effective dose for metabolic benefits.
We haven't done that (yet), but Søberg's research at the University of Copenhagen in Denmark is a start. She recently established that as little as 3-4 minutes of winter swimming each week was sufficient to recruit and maintain active brown fat -- no matter whether it came all at once, or in several brief sessions (Søberg et al. 2021).
The fjords of Denmark are very cold in the winter, typically less than 40F. Assuming these are cold-acclimated swimmers (thermal comfort of 60F), the time under delta-temperature measurement approach suggests that her study subjects were getting about 220 degree-minutes of time under temperature cold dose every week. If your thermal comfort line is closer to 70F, then to get the same cold dose as Søberg's swimmers, you need do only about 7 minutes a week at 40F. As your cold training improves, you have to either drop your temperature or increase your time.
That's why owning a Morozko is so important to serious ice bathers. At first, they might start with a 45F cold plunge, get a good gasp response, and feel the norepinephrine and dopamine response that comes with several minutes in the cold water. However, as their body adapts, they typically discover that they need colder and colder temperatures to achieve the same effect.
Only a true ice bath that can reach 32F can provide that.
Protocols for Cold Plunge Dosing
To test whether this measure offers you any guidance, establish your own thermal comfort line and then test the dose measure against your subjective experience of cold. Also record your objective measures of response such as required time for rewarming, and (if applicable) time before the onset of shivering. If the measure is useful, you might find that you get better at predicting how you will respond to different temperatures of water, and be able to tell the friends you introduce to your Morozko what they could expect, too.
Remember that as you get better cold trained, shivering is no longer a reliable indicator of dose, because your body has built better thermoregulatory mechanisms that make you more tolerant of the cold. However, the first rule always applies.
To get the psychological benefits of the cold, you must plunge at temperatures cold enough to frighten you.
References
Søberg S, Löfgren J, Philipsen FE, Jensen M, Hansen AE, Ahrens E, Nystrup KB, Nielsen RD, Sølling C, Wedell-Neergaard AS, Berntsen M. Altered brown fat thermoregulation and enhanced cold-induced thermogenesis in young, healthy, winter-swimming men. Cell Reports Medicine. 2021 Oct 19;2(10).
About the Author
Thomas P Seager, PhD is an Associate Professor in the School of Sustainable Engineering at Arizona State University. Seager co-founded the Morozko Forge ice bath company and is an expert in the use of ice baths for building metabolic and psychological resilience.