A: Shiver sometimes.
Summary
Human bodies are equipped with two mechanisms of cold thermogenesis: 1) shivering in muscles, and 2) non-shivering in brown adipose tissue (brown fat).
Brown fat shrinks as we age, until shivering dominates.
Shiver when you're new to cold. It takes a week to acclimate.
Resisting the shiver reflex may help increase heart rate variability (HRV) and build psychological resilience, but not all shivering is for thermogenesis.
Sometimes, shivering is for trauma release.
Thermogenesis
There are two mechanisms of thermogenesis by which human bodies generate the heat they need stay warm in the cold:
muscle shivering, and
non-shivering thermogenesis.
Both are considered involuntary responses to cold exposure, initiated by the central nervous system. However, experienced cold plungers report that they have some control over their shiver response. By relaxing their muscles and focusing on their breath, most people discover that they can postpone, if not entirely suppress, the shiver reflex -- or encourage it. Moreover, Wim Hof describes activating his own brown fat just by focusing his thoughts. (Carney 2017, p17). To some, this proposition might seem far-fetched, but others under Hof's tutelage have reported similar experiences.
Hof's contention gains credibility outside his direct sphere of influence when we consider the extraordinary case of the British open-water swimmer Lewis Pugh. In 2007, Pugh became the first person to swim the Arctic Ocean at the North Pole. Years later, under the medical supervision of Dr. Tim Noakes, Pugh was training to attempt a record-breaking Antarctic ocean swim (Pugh 2013). While monitoring Pugh's vital signs, Noakes noticed that Pugh's core body temperature began climbing as much as 2F before Pugh even entered the cold water.
What else but Pugh's own thoughts could have caused his core body temperature to rise? He wasn't sick and he wasn't shivering, so there must've been something in his subjective experience that activated his metabolism. To describe this phenomenon, Noakes coined a new term called anticipatory thermogenesis, in which he speculated that Pugh's brown fat was activated by his thought of cold exposure. And if that's true for Pugh, why couldn't it be true for Hof, Scott Carney, and others experienced with the cold?
Given the experiences of Pugh, Hof, and other advanced cold plungers, we must conclude that the two dominant mechanisms of thermogenesis, skeletal muscle shiver and brown fat activation, are not as involuntary as scientists and medical doctors used to think. In fact, a new study shows that eleven study participants without cold exposure expertise could delay the onset of shivering during whole-body cool air exposure at 10°C. The researchers concluded that "humans can both constantly and periodically suppress shivering activity, leading to a delay in shivering onset and a reduction in shivering intensity" (Arnold et al. 2022).
Given the fact that we can exercise some control over our shiver response, we must give some consideration to whether we should.
The reality of that choice probably explains why I often get the question, "Should I shiver?"
Skeletal muscle shivering
We all know what it's like to have our muscles tremble in the cold. Often, it begins with chattering teeth, but if you're like me, you also know the feelings of your fingers, hands, and arms all shaking uncontrollably when you've been in the ice bath too long. For me, the shivering often happens during afterdrop -- i.e., when I'm trying to rewarm, and the blood is returning to my skin and the restored circulation is balancing cold and warm sections of my body.
Shivering is a normal, healthy reaction to heat extraction. As we lose heat to our cold surroundings, our bodies activate trembling in our muscles, where mitochondria inside our muscle cells convert glucose and triglycerides from our bloodstream into ATP, fueling rapid contraction of muscle fibers. Shivering is mechanically inefficient, and that's the point. Muscles can only generate heat by moving. But with nowhere to go -- i.e., no real exercise or work to do -- the muscles of the body resort to shivering, just to keep the body warm.
Muscles are the greatest source of thermogenesis in humans (Blondin & Haman 2018). In warm environments, heat production in muscles is problematic. For example, exercise can increase heat production up to 20 times the basal metabolic rate, and the excess heat can damage mitochondria. In Precool Your Workout in an Ice Bath, I explained how extracting heat produced by working muscles is effective for boosting peak power output and postponing fatigue.
However, exercise is voluntary. We typically think of shivering as an involuntary response initiated by the central nervous system. When temperatures drop, thermoreceptors in the skin and/or body core signal the hypothalamus in the brain, which activates the sympathetic nervous system to command the muscles to shiver.
The hypothalamus is located in the center core of the brain, directly above the brain stem. Evolutionarily, it is one of the most primitive aspects of our brain anatomy. It likely originated long before evolution of vertebrate animals, and consequently it governs the most primal human functions, including appetite & thirst, sex drive, sleep, digestion, and thermoregulation.
It is already widely accepted that activity in the hypothalamus controls thoughts and motivates behaviors. What is sorely under-researched is the hypothesis that thoughts and behaviors might control activity in the hypothalamus. Nonetheless, extreme cases like Hof and Pugh suggest this may be the case.
Thus, two things could be happening when you relax your shivering reflex in the ice bath:
You accelerate the drop in your skin & core body temperature by interrupting shivering thermogenesis, and
You practice conscious control of central nervous system functions that are ordinarily involuntary.
Both can be useful functions. The first might amplify activity in your brown fat, and stimulate your body to recruit more. The second might strengthen vagal tone, improve emotional regulation, increase heart rate variability, and enhance constructive adaptation to stress.
Non-shivering thermogenesis
In How To Increase Brown Fat I wrote about how brown fat declines with age and why we would want to retain it. In short, increased levels of brown fat are associated with reduced risk of metabolic disorders & related diseases (including Type 2 diabetes), improved body composition, healthier livers, reduced risk of heart disease & cancer, and better mental health. Unlike white fat, which stores energy, brown fat burns it. And although some foods like hot peppers, or herb extract likes berberine, will activate brown fat, deliberate cold exposure is best way to activate and recruit it (Saito et al. 2020).
What I haven't written about yet is how much cold exposure you need, and how long it takes before you know your body has started adding brown fat.
Mitochondria in the cold
The principal physiological difference between white and brown fat is the density of mitochondria inside each cell. Brown fat are packed with thousands of mitochondria in each cell, to meet the demands of cold challenge. By contrast, white fat cells have comparatively few.
The distribution of mitochondria in each type of fat is consistent with their different functions. For example, during non-shivering cold thermogenesis, white fat cells release stored lipids to be carried by the bloodstream to the brown fat cells, where they are oxidized by mitochondria to produce heat. Therefore, more mitochondria are found in the brown fat cells, because they are the locus of energy conversion.
The process of recruiting new brown fat requires synthesis of new mitochondria, called mito-biogenesis, to populate the new brown fat cells. However, given the fact that mitochondria have their own DNA, separate from the DNA in the nucleus, and possibility that replication of millions of new mitochondria might introduce copying errors or mutations that degrade mitochondrial function, it may interest us to know what sort of quality control procedures are in place to keep our mitochondria functioning like they did when we were young.
The process by which the body removes defective mitochondria is called mitophagy. Defects in mitophagy result in mitochondrial disorders that are associated with a myriad of diseases, including Alzheimer's.
Both mitophagy (removal of defective mitochondria) and mito-biogenesis (production of new mitochondria) take place in response to deliberate cold exposure (Yu et al. 2018). In other words, cold exposure doesn't just increase the overall level of mitochondria, but also improves the quality of mitochondria. In that respect, it may slow or reverse ageing.
Cold acclimation
Studies in mice and humans indicate that a transition from skeletal muscle shivering to non-shivering thermogenesis in brown fat can take about a week. To demonstrate the transition, researchers at the University of Ottawa studied seven men in their twenties who lacked acclimation to the cold.
Researchers measured their shivering response to acute cold exposure (administered via liquid cooled vest). Then, to acclimate subjects to the cold, they immersed them up to their clavicles in 57F cold water for one hour, for seven days straight, and retested the subjects under acute cold challenge using the vest.
They discovered that cold acclimation significantly delayed and reduced shivering, while total heat production stayed constant. The implication is that just seven days of cold acclimation was sufficient to shift thermogenesis from skeletal muscles shivering to non-shivering thermogenesis in brown fat (Gordon et al 2019).
Cold acclimation is accelerated by a process called bieging, in which white fat cells adapt by synthesizing additional mitochondria and breaking up the lipid reservoir within the white fat cell into smaller droplets. These beige cells have some characteristics of white fat, some characteristics of brown fat (Cedikova et al. 2016), and are consequently able to support non-shivering thermogenesis.
The Ottawa results corroborate the findings of separate study that showed ten days of cold acclimation increased activity in brown fat, improved subjective measures of thermal comfort, and reduced self-reported shivering (van der Lanset al. 2013). That helps explain why the elderly have a reputation for feeling cold all the time -- even at temperatures young people consider too warm. Brown fat shrinks with age, so older people are more likely to rely on shivering, which is less comfortable. It's not yet clear whether loss of brown fat is an inevitable consequence of age (Zoico et al. 2019), or due to an accumulated deficit in cold exposure in industrialized countries that causes maladaptation of fatty tissues. In either case, it may be that deliberate cold exposure is able to slow the loss of brown fat, which may delaying risk of some of the diseases associated with old age.
Trauma trembling
Not all shivering is related to thermogenesis. Keeping in mind that the central nervous system and the hypothalamus play a critical role in the regulation of emotions, it may come as little surprise to learn that shivering can also be a response to trauma.
I first addressed the question of shivering for trauma release at the end of an article called Ice Bath & Trauma. There, I introduced the idea that an ice bath could prime subjects for PTSD therapies by normalizing their cortisol levels. Although cortisol is known as the stress hormone, I cited studies showing that people with PTSD suffer from chronically low cortisol levels, and that boosting cortisol prior to therapy can accelerate resolution of the trauma.
Shuddering, trembling, shivering
Peter Levine describes his personal experience with "shuddering" and the physical trauma of being run over by a car in his classic In An Unspoken Voice: How the Body Releases Trauma & Restores Goodness (Levine 2010 p5). Lying on the pavement, conscious but immobilized, Levine remembers:
A trembling wave of release moves through me, and I take my first deep breath. Then a jagged shudder of terror passes through my body. Tears are now streaming from my eyes... . I am sucked down by a deep undertow of unfathomable regret. My body continues to shudder. Reality sets in. In a little while, a softer trembling begins to replace the abrupt shudders. I feel alternating waves of fear and sorrow... . I feel a flicker a hope, then a rolling wave of fiery rage. My body continues to shake and tremble, It is alternately icy cold and feverishly hot... . As I continue to gently tremble, I sense a warm tingling wave along with an inner strength building up from deep within my body.... . The ambulance paramedic takes my blood pressure and records my EKG... . A minute later she tells me that my heart rate is 74 and my blood pressure is 125/70... . I breathe a deep sigh of relief. "Thank you," I say, then add: "Thank God, I won't be getting PTSD." My shaking and following my self-protective responses (to move my arms) helped me to reset my nervous system and brought me back into my body. - Peter Levine, PhD
According to Levine's work, immobilization is one of most important risk factors that turns a stressful experience to PTSD. His experience as a clinician working with PTSD patients has convinced him that trauma is held in the body, as if the sufferer is unable to escape the "freeze" activation of the sympathetic nervous system. He writes:
I once had the opportunity to work with a patient who had been in a catatonic state for two or three years... . I spoke to him softly about the shaking and trembling that I observed with people and animals when they come out of shock. I had also talked with the chief psychiatrist, and he agreed that they would not give him an injection of thorazine (or straightjacket him) if he came to in an agitated state... . Two weeks later I got a call from the psychiatrist. The man begun to shake and tremble, started to cry and was released to a transitional living situation six months later. - Peter Levine (ibid. p62).
An analogous phenomenon has been observed in patients recovering from anesthesia. In post-operative shivering, patients experience tremors as the effects of the anesthesia wears off. Because core body temperature can drop during surgery, and sedation precludes shivering, most medical professionals attribute post-operative shivering to hypothermia. However, studies consistently demonstrate that core temperature has only weak predictive value in identifying patients likely to experience shivering during recovery. In fact, a stronger predictor is young age, given that children are more likely to shiver after the trauma of surgery than adults (Torossian et al. 2005). Given that children typically have more and larger brown fat depots than adults do, that finding is counter-intuitive if poor thermoregulation is the cause, .
However, surgery on the limbs or joints presents a greater risk for post-operative shivering than others. From Levine's experience with immobilization, this makes sense.
Children can understandably be traumatized by experiences that might amount to little more than a nuisance for adults. Levine says that "children being treated for injuries requiring immobilization, such as splints, casts and braces" were particularly at risk of "shell-shock." One study showed that among all orthopedic patients being treated for broken bones, between one-third and one-half develop post-traumatic stress disorder (PTSD) that persists long after the physical injuries heal (Vincent et al. 2015).
These studies suggest that the post-operative shivering, at least in some cases, is not the result of hypothermia, but the body's own instinctive effort to release the trauma of surgery and immobilization and reset the central nervous system.
Breathless
One characteristic, autonomic response to entering an ice bath is the gasp reflex. At the moment the body first experiences the shock of the cold water, an involuntary impulse causes a sudden inhale, typically followed by temporarily holding the breath.
You might guess that the gasp reflex is the body anticipating that the mouth and nostrils will eventually be submerged, and commanding the lungs to take in one last breath before plunging fully underwater -- except there is a separate and conflicting phenomenon called the dive reflex that slows the heart rate and better prepares the body for full submersion when cold water reaches the face. We can't rule out the possibility that the gasp and dive reflexes are meant to work together in some complex, consecutive fashion but a different explanation informs the question of shivering.
All traumas leave us breathless in some way. -- Peter Levine (2010 p213).
When I asked Patrick Porter, PhD , neuropsychologist and creator of BrainTap, about breather and trauma he described an example of how trauma can leave us breathless.
"Remember how you might comfort a children who has been hit by a pitch in a baseball game?" he said. "They're scared and sobbing, and what does the coach say? He says 'Breath.' "
Descending into the ice bath activates the sympathetic division of the autonomic nervous system -- the same circuits that create the fight, flight, freeze, or fawn response to stress. You can see Porter's gasp reflex in the accompanying video of his first ice bath. He's being coached by Jerame Mudick, a certified Wim Hof Instructor and an experienced cold guide.
Mudick's advice? Breathe.
Ice bath for anxiety?
My own experience of the ice bath is now more relevant to my psychology than to my metabolism. I've already reduced my Prostate Specific Antigen (PSA), and elevated my testosterone. Despite still being overweight, I'm metabolically very healthy.
Given my practice of daily ice baths, I can only assume that I'm cold-acclimated with plenty of brown fat, and that my mitochondria are in great shape.
But there are days when I go into my ice bath and start shivering in the first 45 seconds.
What's going on?
Those are the days when I'm stressed. Maybe I'm feeling anxious, or lonely, or I've just had an argument with a loved one, or I'm feeling inadequate in some way.
Those are the days when I let the shiver come. I welcome the possibility of the stress release and (as Levine says) the reset to my central nervous system. While it is true that it's impossible to stay in a low mood when cold water therapy is giving you a 3x dopamine boost, it may be that, like Levine, shivering resets my central nervous system and saves me the trouble of carrying an unresolved stress experience into the rest of my day.
Ice Bath Shiver Protocols
When to allow shivering? When to resist the urge?
When you're beginning your cold therapy practice, don't worry about shivering. When you feel the shiver reflex, you know you've activated your sympathetic nervous system, put yourself into thermal deficit, and your body has started thermogenesis. Like the subjects in the cold-acclimation studies, your body has begun the process of beiging white fat and recruiting brown fat. Between seven and ten days later, you will have shifted your thermogenesis from shivering to non-shivering dominant.
Once you've become acclimated to the cold, and you have non-shivering thermogenic capacity, resisting the shiver reflex with structured breathing will strengthen your parasympathetic nervous system, increase your heart rate variability and strengthen your vagal tone. By practicing interoception -- i.e., increasing awareness of your body and the sensations or feelings within it -- and using your shiver reflex as biofeedback, you will build psychological resilience against stress (Pinna & Edwards 2020).
However, on the bad days, when anxiety has got a grip on your emotions, or you have an audition, a major presentation, or difficulty of overcoming a loss, a failure, a mistake, or a major rejection, and you feel the weight of shame on your shoulders...
... those are the days when you can set an intention for your ice bath: "Today, I am here to shiver."
Your nerves might thank you for it.
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.