Cold and Hot

We evolved with weather. We do not live with weather anymore. The modern human spends nearly all of their waking hours inside a climate-controlled box held at seventy degrees, drives between climate-controlled boxes inside a climate-controlled vehicle, sleeps in a climate-controlled bedroom under a climate-controlled blanket, and then wonders why the body has lost its ability to regulate its own temperature. The body that never experiences thermal stress forgets how to handle thermal stress. Like every other system in the body, the rule is: use it or lose it. Most adults have lost it.

The fix is to put the body back into the conditions it was built around — heat that makes you sweat, cold that makes you flinch — and to alternate between them deliberately, on purpose, frequently. We call our version of this the Contrast Boot, and we do it before every leg day and every other major workout. It’s the cheapest, most underrated piece of health work I know.

Our house burned down. We are training at Lifetime Fitness until the new house is built, which has the unintended benefit of an excellent spa, a cold plunge, a steam bath, and a sauna right next to the weight room. The standard Contrast Boot is fifteen minutes in the spa followed by five minutes in the cold plunge. That is it. Twenty minutes. Then we lift.

Once a week I run a deeper version — a medley of spa, steam bath, and sauna with cold plunges in between — that takes about an hour. The deeper version is the dose that pushes the adaptation; the daily version is the maintenance.

Effie does the standard Contrast Boot with me. She does not do the deeper medley. She does not need to. The standard dose, done consistently before training, has rebuilt thermoregulation in a body that the disease had spent years stripping it from.

What heat does

Sauna and hot water are not just relaxing. They are mild, controlled, repeatable hyperthermia, and the body responds to repeated hyperthermia by producing heat shock proteins — molecular chaperones that stabilize other proteins, repair damaged ones, and tag the broken ones for disposal. HSP70 and its cousins are some of the most studied longevity-relevant molecules in biology, and the most reliable way to produce them is to make yourself hot, regularly, on purpose.

The Finnish data on regular sauna use is some of the cleanest population-level health data anyone has on a single intervention. Four to seven sauna sessions per week are associated with roughly a forty percent reduction in all-cause mortality, large reductions in cardiovascular death, and meaningful drops in dementia incidence. The Finns have been doing this for a thousand years. The rest of us are figuring it out.

For someone whose lifting capacity or aerobic capacity is constrained — which describes most people with chronic disease, including MS — sauna is also a substitute pathway to cardiovascular adaptation. Heart rate climbs. Plasma volume expands. Blood vessels become more responsive. You get many of the same vascular benefits as a hard cardio session without having to crush a body that may not be ready for hard cardio. This is a real workaround, and it is criminally underused.

What cold does

Cold does almost the opposite, and that is the point. Cold exposure triggers a massive sympathetic response — a sharp norepinephrine spike, brown fat activation, mitochondrial biogenesis, the production of cold shock proteins like RBM3 — and then, after exit, a sustained parasympathetic rebound that leaves you calmer, sharper, and noticeably warmer to your own body than you were going in.

The most consequential thing cold does, for Effie and for a lot of people, is dopamine. A single cold plunge produces something on the order of a two-and-a-half times sustained dopamine elevation lasting for hours after exit, with no crash on the back end. This is one of the few reliable, drug-free, free things on Earth that does this. Effie started doing the cold plunge for the dopamine, plain and simple. The fact that it also retrains the autonomic nervous system, improves mood, sharpens focus, and pulls fluid back into central circulation is bonus.

For Raynaud’s syndrome — which Effie has, and which is a vascular dysregulation where the small vessels of the fingers and toes clamp shut at the slightest cold and refuse to reopen on a sensible schedule — the conventional advice is “stay warm.” That advice, like most conventional advice, treats the symptom and ignores the system. The system is broken because the autonomic nervous system has stopped responding correctly to thermal input. The fix is not to remove thermal input. The fix is to give the system progressively more thermal input, under controlled conditions, until the responses come back online. The cold plunge is the controlled input. After months of it, her hands work better, her fingers stay pink in conditions that used to turn them white, and the cycle the disease was reinforcing is being unlearned.

The Effie thermometer

The clearest sign of how much this has worked for Effie is the temperature she can now tolerate, in either direction.

She used to get knocked out by seventy-four degree weather. Seventy-four. A breeze on a spring afternoon would put her on the couch for the rest of the day, because MS heat sensitivity — the textbook Uhthoff’s phenomenon — was firing on a body whose thermoregulation had collapsed. Demyelinated nerves slow down or stop signaling whenever core temperature ticks up even slightly. The standard MS playbook is to avoid heat. The standard MS playbook would have shrunk her livable world down to the narrow band of climate-controlled indoor air that the disease tolerated, and then it would have shrunk further every year as the tolerance window got narrower.

We did the opposite. We took her into the hot water, on purpose, repeatedly, every other day, and we let her body recalibrate. She now does great in deeply hot water — water that would have been unthinkable two years ago. She walks outside on summer days without her body shutting off. She is not “managing” Uhthoff’s anymore; she has retrained the system that was producing it. The disease did not get smaller. The window of conditions she can live inside got much, much bigger.

This is the survival case for hot and cold work, and it is why I view it as non-negotiable for an MS body. The default trajectory of the disease is to keep narrowing the world around the patient until the world is a single climate-controlled bedroom. Every degree of thermal range you reclaim is a piece of the world handed back. There are not many pieces of life worth fighting harder for.

Why we do it before workouts

The Contrast Boot is not a recovery tool, though it does help with recovery. We do it as a primer. The reason is simple: by the time you walk out of a cold plunge, you are alert in a way you cannot fake, your dopamine is elevated, your vascular system is wide open, and your nervous system is in exactly the state you want for a heavy lift. You are sharper. You are more focused. You hit the bar with intent that is hard to summon cold. The leg day that follows a Contrast Boot is a different leg day than the one that follows a cup of coffee and a slow drive in.

For Effie specifically, the dopamine bump from the cold is the difference between training as a chore and training as something her brain actually wants to do. She is not white-knuckling through workouts on willpower. She is showing up high on her own chemistry, which is the only way training survives long enough to compound.

We did not evolve for eternal summer

Step back and look at the bigger frame.

The human body was built to operate across a wide range of temperatures, which is a strange thing to have to remember. We evolved standing in cold rivers, walking across hot savannas, sitting around fires at night, sweating through afternoons, shivering through dawns. The body has hardware for all of this — sweat glands, brown fat, vasoconstriction, vasodilation, hormonal cascades that switch within minutes — and the hardware atrophies when it is not used, exactly like a muscle.

We have built a civilization that uses the hardware almost never. We commute from a sixty-eight-degree house to a seventy-degree office in a seventy-two-degree car. The eternal-summer indoor climate is not normal. It is a recent invention. And if the power goes out, if the AC fails in a heatwave, if the heat fails in a winter storm, if you have to be outside for any extended period — your body will be tested, and your body will not be ready, because your body has not had a chance to be ready in years. This is not theoretical. People die in heatwaves. People die in cold snaps. The deaths are not random. They are concentrated in bodies that have lost the ability to handle the temperatures their species evolved to handle.

The Contrast Boot is, among other things, insurance. It is the practice of keeping the thermoregulatory system rehearsed. It is the practice of refusing to let the box at seventy degrees become the only place your body can live. There will come a day when the box is gone, briefly or permanently, and you will want a body that can handle that day. You build it now.

You don’t need a Lifetime membership. You need warm water, cold water, and the willingness to alternate.

Basic version: end every shower with a minute of the coldest water the tap will produce. That’s the entire intervention. Build up over weeks. After a month you’ll tolerate a longer dose. After three months you’ll look forward to it.

Intermediate version: find a pool, a hot tub, a sauna, or a public bath house with a cold plunge. Twenty minutes total. Mostly hot. End on cold. Two or three times a week.

Advanced version: the Contrast Boot. Fifteen minutes hot, five minutes cold, before training. Add a deeper sauna session once a week if you have the access.

For an MS body, or anyone with autoimmune dysregulation: start gentle, work up slowly, and trust that the system can be retrained even if your textbook says it can’t. Effie is the proof. She started where most MS patients are told to stay; she didn’t stay there. She’s now living in a much wider thermal world than the disease had agreed to give her.

The body didn’t evolve for eternal summer. Stop pretending it did. Get hot. Get cold. Get back the range you were born with.