How cold should the ice bath be?
An ice bath will improve your HRV, but will a cold plunge?
Non-shivering thermogenesis is signaled in the brown fat by sympathetic nervous system activation.
Temperatures that fail to frighten you — i.e., activate your fight-or-flight response — might fail to stress your nervous system in ways that lead to hormetic improvement in physiological markers of psychological resilience.
Heart rate variability (HRV) is the principal measure of adaptability & resilience to stress.
Once you become acclimated to cold plunge temperatures, only an ice bath will be sufficient to stimulate hormesis.
Heart rate variability (HRV) is your ability to withstand and be resilient to the human stress response. – Jay Wiles PhD, Hanu Health Chief Scientific Officer
What is heart rate variability (HRV)?
There are hundreds of scientific studies supporting the belief that psychological and physiological health are interrelated. Nevertheless, what most people don’t know is that psychological distress leaves physiological markers that can be measured before the negative effects become acute.
Heart rate variability (HRV) is one of the most important physiological measures of mental health (Hartmann et al. 2019).
When HRV declines, emotional self-regulation, psychological flexibility, and resilience to stress also decline. According to researchers at the University of Sydney, Australia “over the longer-term, reduced HRV leads to immune dysfunction and inflammation, cardiovascular disease and mortality, attributable to the downstream effects of a poorly functioning cholinergic (i.e., parasympathetic) anti-inflammatory reflex.” (Kemp & Quintana 2013).
By contrast, increasing HRV protects against low mood, depression, anxiety and stress. While HRV relates to the heart, it is not a reliable measure of cardiovascular health. Unlike resting pulse (heart rate), or blood pressure, or V02max, heart rate variability is a complicated measure of psychological adaptability, rather than a measure of cardiovascular health.
Changes in breath, mood, metabolic demands, and nervous system activation demand changes in operation of the heart, such as speeding up or slowing down. A constant heart rate is one that fails to adapt to changing needs, which is why higher HRV is generally considered healtheir.
Measuring variability requires comparing the amount of time between heart beats (Figure 1, below) and performing a statistical analysis of the results. Although it may be counter-intuitive to think that an irregular heart beat indicates a more resilient heart, the fact remains that the more variation in the heart beat, the more adaptable the heart. Constancy can be an indication of poor resilience to stress.
HRV is calculated by measuring the change in time between the beats, to assess how well the heart can adapt to changing needs of the brain and body (Nkurikiyeyezu, et al. 2020).
Resting HRV is a better measure of mental health than physiological health. Because the signals that control heart rate originate in the autonomic nervous system, HRV is a marker of nervous system resilience, rather than cardiovascular strength. A resilient nervous system will engage in a continuous process of sensing its environment, anticipating demands, and adapting bodily functions.
All of these functions are closely connected to the brain, so it may not be a surprise to learn that HRV is correlated with cognitive function. According to a recent scientific review by researchers in Italy, increased resting HRV is associated with better reasoning, improved memory, better grasp of language, increased attention, and improved executive function “to respond quickly and flexibly to environmental demands” (Forte et al. 2019).
Can cold exposure improve HRV?
According to Hanu Health Chief Scientific Officer Jay Wiles, PhD, there are several good health practices that will boost HRV, including: exercise, quality sleep, good nutrition, meditation, and loving relationships (Wiles 2022). However, a number of recent studies have revealed:
Uncomfortable cold exposure is a reliable way to improve heart rate variability (HRV).
In Can Cold Water Cure Depression? I wrote about how cold water swimming generates endogenous dopamine and norepinephrine that make it impossible to stay in a low mood. And in Stress Inoculation I wrote about the benefits of using your ice bath to create eustress — i.e., beneficial stress that strengthens you, rather than distress that weakens you.
However, I never explained the physiological mechanisms by which cold plunge training will build psychological resilience to depression and stress.
As you may have already guessed, HRV is the link between deliberate cold exposure and resilience.
When an international team of researchers from France, Spain, and the United States sought to investigate cryotherapy and HRV, they discovered a reliable dose-response relationship. They exposed thirty healthy men to three minutes of “cryostimulation” using cold, dry air at temperatures between -10°C and -160°C, then compared several physiological markers, including HRV, to a control group that was not exposed to cold. They discovered that:
pulse went down (HR, below), and
HRV went up (as measured by root mean square standard deviation, RMSDD & high frequency band, HF).
Moreover, when the head and neck were included in the exposure (WBC in figure below) effects were stronger than when they were protected from cold (PBC).
Cryostimulation in very cold air reduced heart rate (HR) but increased heart rate variability (RMSSD, HF) in healthy men (Hausswirth et al. 2013).
How cold does an ice bath have to be?
An extensive body of research indicates that the metabolic benefits of deliberate cold exposure are available at temperatures not much colder than 60°F. For example, German researchers reversed Type 2 diabetes in middle-aged and elderly men (Hanssen et al. 2015). The subjects, from 49 to 65 years old, were all overweight and taking metformin for chronic hyperglycemia.
When researchers put them in a room cooled to 14–15°C (57-59°F) for 10 consecutive days, for increasingly long periods (6hrs on days 3 through 10), they discovered significant improvements in insulin sensitivity without changes in diet or exercise. Moreover, as a group the subjects reduced triglyceride levels by more than 10%, despite gaining a little more than a pound in body weight.
That is, although the subjects did not lose weight while practicing mild cold exposure, they enjoyed a significant improvement in their metabolic health and a reduction in the markers of risk for cardio-vascular and heart disease.
Two things about the study are particularly interesting:
Research subjects who spent 10 days lounging for hours at a time in a room cooled to less than 60F reported their thermal comfort levels. Few felt “cold,” although many sometimes shivered (Hanssen et al. 2015)
Under such mild temperatures, the subjects experienced big improvements in metabolic measures, despite the fact that they did not recruit new brown fat. Careful measurements before and after the 10-day cold exposure study revealed that as a group, the subjects gained little (or no) new brown fat. Other studies have emphasized the role of brown fat activation in consuming glucose and triglycerides (Bartelt et al. 2011), so it might stand to reason that the improvements in blood glucose and lipid profile resulted from subjects who recruited new brown fat. However, this sample demonstrates significant benefits are available at mild temperatures that do not induce shivering, and do not result in new brown fat (at least over the course of the 10 day exposure period).
The participants in the study barely even felt cold. By day 10, most reported being “slightly cool” at the three hour mark, and “cool” at the end of the six hours. Although most of them reported shivering at least “sometimes,” none reported that they were very uncomfortable.
Most important for the purpose of improving HRV, none of the subjects in the German cold acclimation study reported that they were “very uncomfortable” at any time.
In Set Your Forge to a Temperature That Frightens You, I made the claim that “the psychological benefits (of the ice bath) only kick in when your anxiety does.” Partly that is because “the sympathetic nervous system (SNS) plays a key role in regulation of brown adipose (fat) tissue recruitment” (Zoico et al. 2019). Without activation of the fight-or-flight response in the sympathetic nervous system, your body will not recruit brown fat.
Yet, my claim has yet to be tested in a quantitative study of HRV in relation to temperature. To do that requires a systematic investigation of HRV response as a function of cold dose, which the Germans did not include in their study.
For how long should you ice bath?
Measuring cold dose is complicated. I described a unique method in How Much Cold Exposure Do You Get From Your Ice Bath? that requires knowing your “thermal comfort level,” which I define as the water temperature at which you could be exposed indefinitely. By multiplying the time of immersion up to the neck by the difference between the thermal comfort temperature and the temperature of the cold water, you can calculate the cold dose in units of degree-minutes.
Everyone has a different thermal comfort level, and the longer you practice deliberate cold exposure, the colder your thermal comfort level. I regularly plunge at 34°F, and I’ve discovered that I’m comfortable in 50°F water, immersed up to my neck, for what feels like forever — i.e., 30 or 40 minutes at a time without feeling anxious.
By contrast, most studies of the effects of cold exposure are conducted on subjects who are thermally naive — i.e., they lack experience with deliberate cold exposure. There is a paucity of studies on experienced practitioners. Thus, studies that work at mild temperatures with thermally naive participants may exaggerate some effects that can be expected by cold-acclimated practitioners at the same time and temperature — if only because the cold-acclimated body is receiving a lower dose.
Healthy young men experienced a boost in the LF/HF statistical measure of heart rate variability only when experiencing thermal discomfort (Zhu et al. 2018).
In a study of HRV and cold exposure conducted in China, researchers reported “there were individual differences in thermal comfort, thermal sensation scores and HRV among subjects when exposed to a specific environmental condition, which meant that the subjects might not have the same thermal sensation and comfort scores under the same environment” (Zhu et al. 2018). They discovered that increased HRV only resulted when their research subjects felt uncomfortable. Thus, the subjective feeling of temperature may be more important than the temperature itself.
Because both the amount of cold dose and baseline HRV varies among individuals when time and temperature are constant, a proper study of psychological resilience and ice baths must endeavor to conduct near-continuous monitoring of HRV in individual subjects, at increasing cold dosages. Cross-comparison between individuals may be misleading, so any improvement in HRV must be established on an individual basis.
The hypothesis is that temperatures must be cold enough to induce anxiety to strengthen adaptive heart response, as measured by HRV, or they will fail to promote formation of increased mental resilience.
While we can imagine an experimental protocol to test this hypothesis, no such study has yet been conducted. Nonetheless, wearable HRV monitoring technology is advancing to the point where it should soon be feasible to carry out the study on a large enough number of people to obtain statistically significant results.
Ice bath vs cold plunge
I’m often asked about how cold the water in a cold plunge must be to obtain the benefits of deliberate cold exposure. For example, some customers will ask me, “Why do I need ice?”
I usually answer, “You probably don’t need ice in your ice bath. But I do.”
For me, it’s not a ice bath experience unless I can see ice floating on top of the water, because that’s how I can see that the water is going to be less than 40°F. Anything warmer than that fails to activate my sympathetic nervous system.
That is, it fails to scare me. And without that sympathetic activation of being frightened, what does my HRV have to work against?
Because HRV is a physiological measure of psychological resilience, it stands to reason that a temperature that fails to activate your nervous system may also fail to improve your HRV.
In this respect, an ice bath performs a function that a cold plunge (for experienced plungers) cannot, because cold plunges do not make ice.
Newcomers to deliberate cold exposure will hardly notice the difference. Because they are not yet cold acclimated, they may experience shock and discomfort at temperatures as high as 50°F. And even if they don’t experience discomfort, like the overweight German diabetic men, they will benefit from challenging their metabolism.
Nonetheless, after acclimating to warmer temperatures, deliberate cold exposure practitioners who are like me will be seeking the extreme cold of the ice bath — not for exercise recovery, and not just for metabolism, but for the psychological benefits characterized by HRV.
Hanu Health promises that they’ll have an Android version of their HRV monitoring software ready for me shortly. At that time, I will commence with my study of cold dose versus HRV.
If you’re already doing both, will you report to me your results?