Finland’s Arctic circle might not seem like a great place to run a marathon barefoot and in shorts—unless you’re Wim Hof. Hof, better known as "The Iceman," has attained roughly two dozen world records by completing marvellous feats of physical endurance in conditions that would kill others. Yet even he was understandably nervous the night before his 26-mile jaunt at -4 degrees Fahrenheit.

“What did I get myself into?” he recalls thinking. But from the moment his bare toes hit the snow, he began to feel “surprisingly good.”

The 59-old Dutchman has climbed Mount Everest in Nepal and Mount Kilimanjaro in Tanzania—Africa’s tallest peak—wearing shorts. “I’ve done about anything I can fantasize about in the cold,” Hof said in an interview. He holds the Guinness World Record for longest swim under ice, and has also endured the extremes of dry heat, running a half marathon through the Namib Desert without drinking any water.

Athletes aren’t the only ones interested in these feats. Now doctors have put the Iceman’s brain—and body— on ice in an effort to better understand the mental and physical mechanisms that allow Hof to seemingly defy the laws of nature. Otto Musik, a pediatrician in Wayne State University’s School of Medicine and his coauthors recently put Hof into a magnetic resonance imaging (MRI) machine while exposing him to cold water and analyzed what happened inside his body.

The results, published recently in a study in the journal NeuroImage, might at first sound more like mumbo jumbo than fact: Researchers found that Hof is able to use his mind to artificially induce a stress response in his body that helps him resist the effects of cold. Musik frames it as a case of “brain over body,” in which Hof activates an internal painkiller function by conducting breathing exercises, then exposing himself to a threat like extreme, sudden cold.

“By accident or by luck he found a hack into the physiological system,” Musik says. He adds that this “hack” allows Hof to feel euphoric while in a freezing cold environment that would be unpleasant in normal circumstances. The researchers tested Hof’s responses alongside around 30 control subjects.

The pediatrician had conducted other research on the way the human body reacts to extreme temperatures. When he heard about a man who sits in buckets of ice cubes for hours at a time and walks up the Himalayas like it was a summer stroll through a wine vineyard, he was intrigued.

Hof attributes his success to what he has dubbed the Wim Hof Method, a type of conditioning that involves a series of breathing exercises he says anyone can replicate. Rather than by luck or accident, Hof says he learned his technique by trial and error while going out into nature: “I had to find the interconnection of my brain together with my physiology.”

The technique first requires relaxation; Hof says he must find a comfortable place to lie down like a sofa or bed. Then he begins a series of deep breathing exercises for several minutes, often prompting a kind of tingling in parts of his body—a sign of hypocapnia, or low carbon dioxide in his blood. “That’s what nature meant us to do, breathe deep when we are stressed,” Hof says.

To a degree, Musik’s research supports Hof’s hypothesis. After Hof went through his preparation exercises to induce this effect, Musik put the Iceman into the MRI machine in a special suit they shot through with shot cold water and hot water in five minute intervals. Some previous research has shown that this exercise makes Hof’s blood more alkaline, since it becomes saturated with oxygen.

Musik found that, when exposed to cold, Hof activates a part of the brain that releases opioids and cannabinoids into the body. These components can inhibit the signals responsible for telling your body you are feeling pain or cold, and trigger the release of dopamine and serotonin. The result, Musik says, is a kind of euphoric effect on the body that lasts for several minutes.

“Your brain has the power to modify your pain perception,” he says, adding that this mechanism is particularly important for human survival. Pain, and the feeling of cold, are basically your body’s way of telling you something is wrong. Since humans instinctively look to remove the source of pain or alleviate any sensation of cold, feeling hurt can help us survive.

But the pain mechanism isn’t always useful. Musik gives the hypothetical example of someone spraining their ankle while being chased by a tiger. Many won’t actually feel the sprain in the thick of the moment since your brain senses the greater danger presented by the tiger. It makes use of opioids and cannabinoids to inhibit pain signals to allow you to run away and save yourself despite the injured foot. “Your ankle is not important in this context,” Musik says.

Rather than being a study of brain over body, the research really parallels other work on how mental training of a particular skill can prompt changes in the brain, says Magda Osman, an associate professor of experimental psychology at the University of London. One study in 2006 looked into the brains of London taxi drivers compared to bus drivers. Taxi drivers had more grey matter in their hippocampus, which is the center of hand-eye coordination skills, ostensibly due to a higher level of navigational skills.

“When we spend a vast amount of time cultivating our mental and physical skills, this translates into neurological differences when compared to those that don't practice these skills to the same degree,” says Osman.

Musik’s study offers a twist to this thinking: It shows that breathing, often thought of as an automatic skill, can be willfully controlled. Harnessing breathing can result in increased activity in the parts of the brain that deal with thought and action, Osman says, which over time can lead to significant physical changes.

Yet stress-induced analgesia, Hof attests, will only last a few minutes at best. For him to continue his ability to resist the feeling of cold, Musik believes that his body needs to anticipate the continued effect, which in turn actually helps his body maintain the state it’s in.

“The placebo effect is real,” he says. “This is actually by generating in your cortex a certain expectation, and this expectation is fulfilled.” He adds that the expectation triggers the release of more opioids, serotonin, and dopamine in a kind of self-fulfilling cycle. In other words, the longer that people jump into holes in the ice practicing this method, the easier it could get as they get more confident in their expectations.

There may not be any pressing need for the world’s population to learn how to run barefoot through the snow, Hof says, but the technique could have other uses. Some of Hof’s patients claim the method has helped them reduce eczema symptoms or other autoimmune conditions.

While he isn't totally clear on the biological processes that help Hof resist frostbite, Musik does thinks Hof’s method might actually suppress tissue swelling and other immune reactions. He and his coauthors write that Hof and his followers’ abilities to take more control of their autonomous systems might have implications in dealing with clinical syndromes.

However, while Hof's method may work for short periods in terms of tricking the mind into ignoring extreme cold, it is less clear how the technique might help human bodies resist the physical effects that one might expect from climbing snowy mountains in shorts. (Nor does it explain being able to survive in the desert without water.)

“You can think whatever you want but your body still freezes and you are dead,” Musik says.