Dr. Craig Heller: Using Temperature for Performance, Brain & Body Health

High-Level Overview

Andrew Huberman talks with Dr. Craig Heller, a Stanford professor of biology and neurosciences and longtime chair of Stanford's biology department, about how the body controls its temperature and how that control can be used to improve physical and mental performance. The central idea is simple and overturns a lot of common practice. The body sheds heat mainly through three "portals": the palms of the hands, the soles of the feet and the upper face. These hairless (glabrous) areas contain special blood vessels that route warm blood straight from arteries to veins. Cooling or warming these portals correctly works far better than the ice packs, cold towels and neck sponges most people use. Heller explains how his lab's palm-cooling research led to a tripling of work volume in the gym, a doubling of endurance in the heat and the near disappearance of next-day muscle soreness.

The tone is curious, technical and practical. Huberman keeps turning the physiology into protocols for athletes, workers in protective gear, people with multiple sclerosis, hypothermia victims and even bulldogs. The conversation moves from lab findings to real-world stories and then to tools listeners can try at home, with side trips into hibernating bears, brown fat, brain freeze and sleep.

The Discussion Explores:

• What cold showers, ice baths and cryotherapy actually do, and the "boundary layer" of still water around the skin
• Why heat, not a lack of fuel, is a fast and major cause of muscle failure
• Why ice on the neck, head or torso can fool the brain's thermostat and even trap heat
• The heat-loss portals in the palms, soles and upper face, and the special blood vessels beneath them
• Palm cooling results: tripled dips, 800-plus push-ups, doubled endurance, and no soreness
• Rewarming people with hypothermia, and cooling dogs
• Palm cooling compared with anabolic steroids, and results in women
• Shivering, brown fat, hibernating bears, brain freeze, fidgeting, caffeine, and temperature and sleep

Deep Dive into Main Ideas

Cold Exposure, Adrenaline and the Boundary Layer

Huberman opens by asking what happens when someone plunges into an ice bath. Heller says the first effect is a big shock and a shot of adrenaline. He thinks this is the main "benefit" of cryotherapy chambers: you feel different afterward, but that doesn't necessarily mean any improvement in physiology or performance. Cold water also triggers vasoconstriction, which, if anything, makes it harder for the body to get rid of heat. In a truly cold bath, though, the whole body surface is exposed, so you lose heat anyway.

He explains the "boundary layer" using a hot bath. At first the water feels almost painfully hot. Then the still water next to your skin comes to equilibrium with it and acts like a blanket, until you move and disturb it. A cold bath works the same way, so staying still means losing less heat, while a shower can never build a boundary layer. For endurance exercise, a few minutes of a cool shower or cold bath before a run raises the body's capacity to absorb the heat it will produce. The body shuts off its own heat loss before core temperature drops below normal, so it doesn't take long. Stanford's cross-country team used cooling before races in hot places. Heller notes that "pacers" will use the advantage to go farther, while others will use it to go faster.

Heat Is Why Muscles Fail

For strength training, the story is local. During heavy sets, muscle metabolism can rise 50 or 60 times over, and heat production rises with it, but blood flow, the only way heat leaves a muscle, cannot keep up. In Heller's words, "you literally have the capacity to cook your muscles." We are only about 20 percent efficient: of the energy in our food, about 20 percent goes into work and the rest is lost as heat. To protect muscle from overheating, the body has a failsafe. A temperature-sensitive enzyme (Huberman names it as pyruvate kinase) that moves fuel from glucose metabolism into the mitochondria shuts off when the muscle gets too hot, cutting off the fuel for ATP production. "That's when you cannot do one more rep."

Huberman is struck by this, since he had assumed failure meant running out of glycogen, yet after three minutes of rest the reps come back even though glycogen couldn't have been restored. Heller calls heat "one limiting factor" and a fast one, adding that heat built up during leg work also affects upper-body fatigue. A cool towel on the thighs doesn't help much, because skin, fascia and muscle are good insulators and heat leaves only in the blood. Drinking ice water helps a little, but its capacity to absorb heat is limited and you can't drink liters of it.

The Thermostat Trap: Why Neck Ice and Cold Vests Mislead

The brain's thermostat sits in the preoptic anterior hypothalamus, and it gets an averaged reading of temperature from the whole body surface. Putting cold on the skin while overheated is like cooling your house by putting a wet washcloth over the thermostat. Heller's lab has watched overheated volunteers cool their body surface, say it feels great, and see their core temperature keep rising. A cooling vest on the torso can even make the heat-loss portals constrict. A cold towel around the neck cools blood heading to the brain through the carotid and vertebral arteries, which protects the brain but makes you feel cooler than you are. You may think you're ready to go again when you've only cooled the thermostat, and when the cooling stops, hot blood from the core returns to the brain.

The stakes are real. Rising temperature impairs thinking: on a treadmill, once body temperature reaches about 39 degrees, people can no longer do simple arithmetic. Heart rate climbs with core temperature ("cardiac drift"), and people heading into heat stroke may paradoxically stop sweating and constrict their blood vessels. Because the warning signs just feel like general misery, motivated people push through them. Heller notes that several high-profile athletic deaths from heat stroke happened during practice, not competition.

The Portals: Palms, Soles and Upper Face

Most of our skin is still "hairy skin" inherited from furred ancestors. The exceptions are the glabrous surfaces: the palms, the soles of the feet and, in primates, the upper face above the beard line. Rabbits use the inner ears and bears use their huge tongues. Under these surfaces lie arteriovenous anastomoses (AVAs), shunts that carry blood straight from arteries to veins, skipping the high-resistance capillaries, so large volumes of blood can pass close to the surface and give off heat. They are described in Gray's Anatomy, Heller says, but for a long time nobody knew what they were for. Warm palms look red, and squeezing a glass turns them white by shutting off that flow.

The practical lessons: don't grip bicycle handlebars (or, Huberman adds, a phone while running) tightly all the time, keep gloves and socks thin when you need to shed heat, and run with loose hands. Heller once ran in Alaska without gloves and found the backs of his hands aching with cold while his palms were sweating. Blood from the upper face also returns through veins that pass through the skull. When you're overheated, flow in those veins reverses, so cool facial blood helps cool the brain. That is why pouring water over a fighter's head between rounds is a natural brain-cooling method. Whether cooling protects against concussion damage is, in Heller's view, controversial but worth studying, since cooling reduces swelling and inflammation.

Tripled Performance and No Soreness

The early story came from Greg Clark, a former Stanford tight end then playing for the 49ers. His usual workout was about five sets of dips, starting with 40. With three minutes of palm cooling between sets, every set beat his control day, he kept adding sets, and his total volume doubled. After a month of sessions twice a week, he was doing 300 dips. "He tripled. Heller also discovered that people who exceed their previous limits with cooling always expect to be sore the next day, and they never are. In a physical conditioning class, the half who cooled reported no significant delayed-onset muscle soreness. Freshman women doing 10 sets of push-ups to failure with cooling reached over 800 push-ups in about 45 minutes, and later joked that he had cost them money because they had to buy new sleeveless dresses for a formal. The gains are real conditioning: You keep your gains," even without cooling later.

For endurance, the first experiment used roughly 18 volunteers walking uphill on a treadmill at about 40 °C, and continuous cooling doubled their endurance. Wearable versions were developed after Ebola workers in Sierra Leone, who couldn't stay in the hot zone in protective gear for more than 15 or 20 minutes, asked for help. Special operations soldiers who tested the technology for the Department of Defense wanted to keep it. One reported it added 20 yards to every club in his golf bag. People with multiple sclerosis, who are very sensitive to heat, have put the device on golf carts and returned to summer golf. The technology is made by a company called Arteria and sold as the CoolMitt (coolmitt.com). It is being used by NFL, NBA, MLB and college teams, Olympic programs and the Navy SEALs while it's refined, and the public can sign up for future availability.

How to Try It: Cool, Not Ice-Cold

The cooling surface should feel cool, not icy. "Ice cold is too cold," because it makes the portals constrict and seals the heat in. Heller standardized on three minutes of cooling, because heat loss follows an exponentially declining curve and the first two to three minutes give the biggest return. For a do-it-yourself version, such as the bag of frozen blueberries Huberman passes between his hands (for about 30 seconds, which he now realizes was probably too short), Heller says it's experimental. His test: after holding the cold item, have someone feel your palm. If it's cold, the vessels have constricted; if it's warm, blood is still flowing. Switching a frozen juice can between hands every couple of minutes might work for a runner.

He also explains heat transfer. Radiation is warming your hands at a campfire; convection is a cool breeze. Heat exchange works best when the medium moves on both sides, blood on the inside and circulating water on the outside, so a still cold pack builds its own boundary layer and loses effectiveness. In a published study, placing the same cold packs on the palms, soles and face cooled hyperthermic people twice as fast as the standard advice of armpits, groin and neck. His analogy: to cool an overheating car, you spray the radiator, not the hoses.

Rewarming Hypothermia, Warming Up, and Dogs

Huberman recounts a morning ocean swim where a very lean teenager became hypothermic. The group, and a skilled lifeguard, warmed his torso and left his hands and feet exposed. Heller explains that his lab entered this field through exactly this problem. An anesthesiologist challenged them over patients coming out of surgery hypothermic and shivering for hours despite warm blankets and heat lamps, because they vasoconstrict and heat can't get in. A prototype that warmed one arm under gentle negative pressure brought the first patient to normal core temperature in about eight or nine minutes with no shivering, and it turned out only the hand mattered. It still isn't standard practice. For rewarming, you can use hotter temperatures on the glabrous skin, such as warm circulating pads on the feet, because the heat is carried away quickly.

On warm-ups, Heller thinks the main value is flexibility and protecting joints, tendons and ligaments, though mitochondria also produce energy less well at low temperatures. Body temperature isn't a uniform 37 degrees: it varies across the day and across the body. Research uses an esophageal probe, ear (tympanic) thermometers are decent if aimed correctly, and the forehead is a reasonable surface site. Knit caps reduce steady heat loss from the head, and helmets should be ventilated because the brain accounts for about 20 percent of resting metabolism. Huberman cools his heat-prone bulldog, Costello, by standing him in cool water, and Heller's group built backpack cooling units with foot pads for sled dogs, which "worked beautifully."

Palm Cooling Compared With Steroids, and Results in Women

Heller's lab analyzed reputable studies of bench press performance with anabolic steroids. They found improvement of roughly 1 percent per week, compared with about a 300 percent increase in a month with palm cooling. He asks why anyone would risk their health and eligibility on "such an ineffective tool." To the objection that the male subjects' testosterone explained the results, the lab ran a comparative study in women and got the same results.

Shivering, Brown Fat, Bears and Other Curiosities

Shivering raises metabolism, but only to about three or four times resting, compared with about ten times for exercise. A fever is essentially "deliberate shivering": the hypothalamus raises its set point. In black bears fitted with implanted sensors and studied with colleagues at the University of Alaska, core temperature drops only to about 33 or 34 degrees during hibernation. On minus-40 days, they shiver to hold that temperature. Ground squirrels and marmots drop to within a degree of their surroundings for seven- or eight-day bouts, then rewarm using brown fat until shivering kicks in at around 15 or 16 degrees. Humans have brown fat, but it is distributed among other fat rather than in discrete pads. Heller says the familiar idea of a brown fat depot between the shoulder blades applies to ground squirrels. Ice on the upper back may still activate brown fat indirectly, by cooling blood in the vertebral arteries so the hypothalamus decides you're cold and triggers a sympathetic response.

Brain freeze may come from direct cooling near the hypothalamus through the roof of the mouth, and more likely from a sharp rise in blood pressure that stretches pain-sensitive blood vessels. Heller mentions, as purely anecdotal, reports that heating with their device relieved some migraines. On fidgeting (non-exercise activity thermogenesis), he notes that about 80 percent of the energy in any movement becomes heat. On caffeine, he points out that working muscle releases adenosine to dilate blood vessels, so an adenosine blocker could in theory reduce muscle blood flow, but he says he can't give an authoritative answer.

Temperature and Sleep

Heller says sleep medicine now emphasizes cognitive behavioral therapy over drugs. That means regular bed and wake times, no screens (rich in blue light) within a couple of hours of bed, time to relax before bed, and managing temperature. A warm bath helps many people. A cool room helps because the thermostat is heading toward a lower set point at bedtime. In a cool room, you can regulate passively by sticking a hand or foot out from under the covers, using the same heat-loss portals, instead of waking up to throw blankets off. Socks can help comfort because the toes are among the most temperature-sensitive spots, a finding from an old study he says was supported by Eddie Bauer.

Conclusion

Craig Heller's message is that temperature is one of the most powerful and least understood levers on performance. The body dumps heat through the palms, soles and upper face, and working with those portals, using cool rather than icy temperatures for about three minutes, can dramatically extend strength and endurance work, prevent soreness, speed rewarming after hypothermia and make heat stroke less likely. The common habits of icing the neck, head or torso can fool the brain's thermostat while the core keeps heating. From tripled dip workouts to golf-cart cooling for people with multiple sclerosis to sleeping with a foot out from under the covers, the episode turns basic physiology into tools anyone can try, though Heller is careful to say which are proven and which are still speculation.

This is a summary of "Dr. Craig Heller: Using Temperature for Performance, Brain & Body Health" from Huberman Lab, giving you the GIST so you can decide if you want the REST. Listen to the full episode.