Using Temperature To Optimize Performance, Brain & Body Health | Dr. Craig Heller

This episode of the Huberman Lab podcast features Dr. Craig Heller, a professor of biology and neurosciences at Stanford whose laboratory works on thermal regulation, Down syndrome and circadian rhythms. The stated focus is how the body heats and cools itself and maintains homeostasis, and how that regulation can be used to improve athletic and mental performance. The introduction makes a strong claim: that learning to control core body temperature is one of the most powerful things a person can do to optimize mental and physical performance, regardless of environment.

The introduction also previews a myth the conversation sets out to correct: that a cold pack on the head or neck is the best way to cool down quickly. According to the introduction, this approach can be counterproductive and lead to hyperthermia. Down syndrome, circadian rhythms and sleep are named as topics left for a possible future conversation.

The discussion opens with cold showers and ice baths, which are often promoted as a general health tonic or as a way to improve resilience or metabolism. The first response described is a shock that produces "a bit of a shot of adrenaline." The guest's answer argues that this adrenaline is the main so-called benefit of cryo chambers, and declines to call it a benefit: a person will feel different afterward, but that does not necessarily translate into any benefit for physiology or performance.

A cold bath or shower also triggers vasoconstriction. The explanation given is that this can make it a little harder for the body to get rid of heat, because the avenues of heat loss are being shut down. In a true cold bath, though, the body's total surface area is large enough that heat is still lost even while vasoconstricted.

Asked which vessels constrict, the answer calls this an area of controversy. It rejects the common picture of the whole skin surface vasoconstricting. Instead, the primary sites of heat loss are identified as the palms of the hands, the soles of the feet and the upper part of the face. These areas sit over special blood vessels that can shunt blood from arteries directly to veins, bypassing the capillaries, which carry nutrients but have high resistance. The answer adds that a handshake can reveal a person's thermal status, depending on whether the hand is hot or cold. A follow-up asks whether handshaking evolved to gauge anxiety. The answer is that the evolutionary origin is unknown, other than perhaps to get the hand away from a weapon.

The difference between a shower and full immersion is described as mainly physical. A person sitting still in a bath develops a boundary layer, which cannot form in a shower. This is illustrated with a hot bath: it feels almost painfully hot at first, then less so as the still water next to the skin comes into equilibrium with it, acting "like having a blanket on you or an insulator on you." Moving disturbs that layer, and the full temperature is felt again. Applied to cold water, staying still means losing less heat, while flailing around increases heat loss.

The conversation separates aerobic from anaerobic activity. In sustained aerobic exercise, heat production rises gradually and spreads through the body until core temperature reaches a level that impairs performance. The benefit described for a cold bath or shower before aerobic activity is that it increases "the capacity of your body mass to absorb that excess heat." A cool shower before a long run, it is said, means taking longer to reach the sweat point and to heat up.

How that advantage shows up depends on the athlete. The answer distinguishes "pacers," who keep the same pace and go farther, from "forcers," who use up the advantage as fast as they can and go faster but not necessarily farther. The benefit is framed cautiously as something that "could be a benefit." As an example, a cross-country team competing in very hot places would warm up and stretch, then extract heat before the race, although the speaker did not know whether the team still does this. The idea is summed up as having "greater scope for heat absorption."

On duration, the answer is "It's not as long as you think. It's minutes." The reasoning is that as core temperature falls, the body eventually shuts off its own heat loss, which keeps temperature from dropping below normal. If warming up has raised temperature by about half a degree, it takes no more than a few minutes to remove that heat, provided the person is vasodilated.

For strength athletes doing sets and repetitions, core temperature rises slowly because only certain muscles are working, but the temperature of those muscles goes up. The effect is described as local. In a compound movement such as barbell squats, the quadriceps, hamstrings and glutes would heat up, while muscles like the biceps are involved only indirectly.

The mechanism offered is a mismatch between heat production and heat removal. Heat leaves a muscle only through the blood. During contraction the muscle squeezes its own blood vessels, so heat cannot leave quickly during work, and it leaves slowly during rest as well. Muscle metabolism can rise 50- or 60-fold during anaerobic activity, and heat production rises by the same factor, but blood flow to the muscle cannot rise 50- or 60-fold.

so you literally have the capacity to cook your muscles.

The broader energetics are explained by comparison to a steam engine. Humans are described as roughly 20% efficient: about 20% of food energy can go into work and the rest is lost as heat. Mammals use that heat to stay well above environmental temperature, but a rise of a few degrees means hyperthermia, and individual muscles can reach hyperthermic limits before the whole body does.

The account then describes a fail-safe. An enzyme critical for getting the products of glucose metabolism into the mitochondria, where ATP is made, is temperature sensitive. When muscle temperature gets above 39 or 39.5, it shuts off, cutting the fuel supply to the mitochondria. This is presented as the point at which "you cannot do one more rep." (The transcript describes this mechanism as protection against damaging muscle "by hypothermia," although the surrounding discussion is about overheating.)

Other limits on muscle activity are acknowledged. Lack of oxygen could matter, but oxygen delivery to muscle is described as pretty good, and running out of glucose would impair performance. The claim is that the rise in muscle temperature is the most immediate cause of muscle fatigue. The answers also avoid overstating it: temperature is repeatedly called "one limiting factor" and "one of them," though "a quick one."

A follow-up asks why, after squatting to failure, a person cannot immediately curl a barbell with fresh intensity if the biceps stayed cool. The answer is that the upper body still has its own fatigue curve, and that curve is influenced by any temperature rise generated by the lower-body exercise. The questioner observes that glycogen cannot be the whole explanation, since a three-minute rest restores the ability to do repetitions without glycogen being replenished in that time, while allowing that other mechanisms may be involved.

Draping a cool towel over the quadriceps after a set does not work well, the answer says, because the body surface is a very good insulator. Even without fur, the skin, fascia and overlying muscle all insulate, which is why heat leaves muscle through the blood rather than through the surface. Standing in front of a fan is grouped with the towel as an approach limited by that insulation.