Chapter 3: Cold as a Tool

Chapter Introduction

In Grade 6 you learned what cold is and how your body responds to it. In Grade 7 you went a layer deeper — vasoconstriction in detail, brown fat, the cold shock response, and the cultural traditions of cold practice across the world.

In Grade 8 you are going to learn how cold gets used — by athletes, by sleep researchers, by adults who have built it into their daily lives. The Penguin is going to teach the research, present the findings honestly, and trust you with the truth: most of what is published on cold has been studied in healthy adult populations under supervision, and the same findings do not always transfer cleanly to a 13- or 14-year-old who has not been screened, trained, or supervised.

That said, you are old enough to understand the framework, read the research with discernment, and start thinking about cold as a tool — something with specific uses, specific costs, and specific safeguards. You are not old enough to be doing serious cold-water immersion protocols on your own. But you are old enough to understand them, so that when you are old enough to do them — and when you have the screening, the training, and the supervision — you can make smart choices.

This chapter has four lessons.

Lesson 1 is cold and athletic recovery — what the research shows about cold-water immersion after exercise, when it helps, when it backfires, and what current science says about its limits.

Lesson 2 is cold and sleep — short and direct. The cool bedroom material lives in Coach Sleep Grade 8. This lesson will cross-reference rather than re-teach, and add only what is Penguin-specific.

Lesson 3 is cold and the autonomic nervous system — also short and cross-referenced. Coach Brain Grade 8 teaches the sympathetic/parasympathetic system in depth. The Penguin adds what cold specifically does to that system and the concept of voluntary discomfort — done carefully, with limits.

Lesson 4 is the math — temperature × time × adaptation as a real framework, what dosing looks like in the research, and how to think about a cold practice across seasons. The Penguin will be specific about what to bring to a parent or doctor if you ever want to talk about starting a practice.

This is the final cold chapter at middle school level. By the end, you will have a clear, science-based framework for thinking about cold — one that respects both its power and its real costs.

Begin. The Penguin is steady.

---

Lesson 3.1: Cold and Athletic Recovery

Learning Objectives

By the end of this lesson, you will be able to:

• Describe how cold-water immersion (CWI) has been used in athletic recovery research
• Identify the typical research dosing (temperature × time) used in peer-reviewed studies of athletes
• Distinguish between cold's effect on subjective soreness and its effect on long-term muscle adaptation
• Recognize that current research has mixed findings on cold for strength training recovery
• Identify when cold may help, when it may not help, and why a 13-14 year old does not yet need it

Key Terms

What Cold-Water Immersion Has Been Used For

Coaches, athletic trainers, and physical therapists have used some form of cold-water immersion (CWI) — ice baths, cold whirlpools, cold tubs — for decades to help athletes recover from hard training and competition. The practice is built around two basic claims:

1. Cold reduces inflammation that follows hard exercise.
2. Cold reduces the subjective feeling of soreness in the day or two after.

Both of these are partly true. The current research is more nuanced than the practice tradition suggests.

The most-studied dosing in the sports science literature has been cold-water immersion at temperatures of 10-15°C (50-59°F) for 10-15 minutes within the first 30 minutes after hard exercise [1]. Some studies use colder temperatures and shorter times. The general pattern: colder = shorter, warmer = longer.

Research consistently shows that this kind of CWI reduces delayed-onset muscle soreness (DOMS) — the achy feeling 1-2 days after hard training. Athletes report less soreness. Subjective ratings of recovery improve. This effect is real and reproducible [2].

The catch comes when you look at long-term adaptation.

The Catch — Cold and Long-Term Strength Gains

Several studies over the last decade have shown that cold-water immersion immediately after strength training can reduce long-term muscle adaptation.

The mechanism is straightforward. When you train hard, your muscles experience small amounts of damage (Coach Move Grade 7 covered this). The body responds with inflammation — which sounds bad but is actually the start of the adaptation signal. Inflammation triggers protein synthesis, the rebuilding of damaged muscle, and the addition of new muscle protein (hypertrophy). Inflammation, in this case, is part of the gain.

When you put ice on a muscle right after training, you suppress that inflammation. The soreness goes down. But the adaptation signal also goes down. A 2015 study by Jonathan Peake at Queensland University of Technology and his colleagues found that subjects who used cold-water immersion after each strength training session for 12 weeks gained significantly less muscle mass and less strength than subjects who did the same training with active recovery (light cycling) instead [3].

Subsequent studies have largely confirmed this finding for strength and hypertrophy training [4]. Cold after lifting reduces soreness but reduces gains.

For endurance training, the picture is more mixed. Cold-water immersion may help some endurance athletes maintain performance across heavy training periods. But for athletes whose main goal is building strength or muscle, the research suggests cold-water immersion right after lifting works against them.

When Cold Might Help an Athlete

Based on the current research, cold-water immersion may be useful for athletes in specific situations:

1. Tournament play, multiple games per day. When the goal is to feel as good as possible for the next event in hours, soreness reduction is the priority. Adaptation can wait.
2. Endurance training blocks. Some studies suggest CWI helps endurance athletes maintain training intensity across heavy weeks.
3. Subjective recovery. If an athlete simply feels better after CWI and uses that improvement to train consistently, the psychological benefit may matter.
4. Heat illness. Cold-water immersion is one of the most effective treatments for heat stroke and severe heat exhaustion — but that is emergency medicine, not training practice.

When cold is less useful or potentially counterproductive:

1. Strength and hypertrophy training periods. The Peake research suggests skipping CWI right after lifting.
2. Off-season general fitness training. No clear benefit, real costs.
3. In place of basic recovery tools. Sleep, food, easy movement, and time matter more.

Why You Don't Need It at Age 13-14

The Penguin is going to be direct.

Most teen athletes do not need cold-water immersion. Coach Move taught you the recovery tools that actually matter at your age: adequate sleep (Coach Sleep Grades 6-8), adequate fuel (Coach Food Grades 6-8), at least one rest day per week, at least 48 hours between hard sessions on the same muscle group, and easy movement on light days. Those tools handle most teen recovery situations.

When teen athletes are not recovering well, the answer is almost never "add cold-water immersion." It is "fix the sleep, the food, the rest day, or the volume." Adding ice to a tired teen who is already short on sleep is patching the wrong leak.

If you are a competitive athlete and your coach is recommending CWI, that decision should involve your parents and a doctor — both because of the cardiovascular questions from Grade 7 and because of the strength-adaptation trade-offs you just learned. The decision is not casual.

The Penguin's read: cold-water immersion is a tool. Tools have specific uses. They have costs. Using a tool because it's trendy is not the same as using it because the situation calls for it. At your age, the situations that genuinely call for CWI are rare.

A Brief Note on Contrast Therapy

Contrast therapy — alternating hot and cold exposure — is a practice with roots in many cultures (Finnish sauna-to-snow, Russian banya-to-cold-water, ancient Roman bath sequences). It has also been studied in sports science as a recovery tool [5].

The basic protocol in research: warm exposure (water around 38-40°C / 100-104°F) for 3-5 minutes, then cold exposure (10-15°C / 50-59°F) for 1-2 minutes, alternating 3-5 times, ending on cold.

The science is still developing. Some studies show contrast therapy works similarly to plain cold-water immersion for soreness; others show modest additional benefit; others show no clear advantage. The mechanism is thought to involve repeated vasoconstriction-vasodilation cycles ("vascular pumping") that may help clear metabolic byproducts of exercise.

The Penguin is going to keep this short here because Coach Hot Grade 8 will go deeper into contrast therapy. The hot side of the equation, the sauna and heat research, and the combined practice are the Camel's territory. The Penguin lays the groundwork; the Camel picks up the rest.

For now: contrast therapy is a real practice with real research behind it, used by trained athletes under supervision. It is not a 13-year-old's home protocol.

Lesson Check

1. What is the typical research dosing for cold-water immersion in athletic recovery studies?
2. Does cold-water immersion reduce subjective soreness after exercise? What does the research say?
3. What does the 2015 Peake study show about cold-water immersion after strength training over 12 weeks?
4. Name two situations in which cold-water immersion may be useful for an athlete.
5. Why does the Penguin say most teen athletes do not need cold-water immersion?

---

Lesson 3.2: Cold and Sleep

Learning Objectives

By the end of this lesson, you will be able to:

• Recall that a cool sleeping environment supports better sleep (cross-reference: Coach Sleep Grade 8)
• Identify why your body's core temperature drops as you fall asleep
• Recognize that cooler bedroom temperatures support deeper sleep stages
• Recognize that this lesson is intentionally short and points you back to Coach Sleep Grade 8 for the detail
• Apply one specific cold practice — a cool bedroom — as the most reliable cold-for-sleep tool available

Key Terms

A Short Lesson — Pointed at Coach Sleep Grade 8

This lesson is intentionally short. The cool-bedroom material lives in Coach Sleep Grade 8 ("Sleep Debt and Recovery"), Lesson 3.1, where you learned:

• The ideal bedroom temperature range for adolescent sleep is roughly 60-67°F (16-19°C)
• Your core body temperature drops about 1-2°F as you fall asleep
• Sleeping in a too-warm room slows or blocks that drop, making sleep harder to start and shallower once started
• A slightly cool shower 60-90 minutes before bed can help by triggering the body to dump heat afterward
• Cooling tools include light bedding, an open window, a fan, and (where available) a cooling mattress pad

You learned this from the Cat. The Penguin agrees with everything the Cat said and is going to add only what is specifically Cold-Coach content: the why in slightly more detail and the practical first step if you want a cold practice that is genuinely useful at age 13-14.

Why Cool Sleep Actually Works

You learned in Coach Sleep Grade 6 that your sleep is run by your circadian rhythm and sleep pressure. As bedtime approaches, your body lowers its core temperature in a controlled way — by about 1-2°F (around 0.5-1°C) below daytime baseline. This drop is part of the sleep onset signal. Your hypothalamus reads "core temperature is dropping" as "we are heading toward sleep" and triggers the cascade that follows.

Your body manages this drop in two main ways:

1. By releasing heat from the body — opening blood vessels in the hands and feet (vasodilation) so warm blood loses heat through the skin. This is why your hands and feet often feel slightly warmer right before sleep, even though your core is cooling.
2. By reducing heat production in the muscles and metabolism.

If your bedroom is too warm, step 1 cannot work well. The blood opening at your hands and feet does not dump heat fast enough because the surrounding air is also warm. Core temperature stays elevated. Sleep onset is delayed. When you finally fall asleep, the depth of sleep is reduced because the same elevated core temperature interferes with stage 3 (deep sleep) and REM cycles [6].

This is also why people who run hot (athletes after evening workouts, kids who recently ate a big meal, people in hot climates) sometimes struggle to fall asleep — their body's core temperature drop is fighting upstream against the ambient temperature.

A cool bedroom solves this. The cool air helps your body shed heat. Your core temperature drops on schedule. Sleep arrives easier and goes deeper.

The Penguin's Practical Frame

If you want one cold practice to focus on at age 13-14, the Penguin's pick is unambiguous: a cool bedroom.

It is:

• Low risk. No cardiovascular stress, no cold shock response, no supervision required.
• Well-supported by research. Decades of sleep science back the 60-67°F range.
• Free. Often just turning down a thermostat or opening a window.
• Cumulative. Works night after night, building up the sleep benefits across weeks.
• Achievable. You do not need to be brave, athletic, or willing to gasp into ice water. You just need a cool room.

The Penguin's take: of all the cold practices available to a middle schooler, the cool bedroom is the most useful. It is also the most under-appreciated. Compared to cold plunges, it is invisible. No social media views. No dramatic gasping. Just a slightly cooler room and slightly better sleep — every single night, across years.

Quiet practices add up. Sometimes the Penguin's favorite cold tool is the one nobody talks about.

What to Bring to the Conversation With a Parent

If your home is currently warmer than 67°F at night and you want to try a cooler bedroom, the conversation with a parent is short:

• "I learned in CryoCove that the recommended bedroom temperature for sleep is 60-67°F."
• "Can we try the thermostat lower at night for a week and see if I sleep better?"
• "If the whole-house thermostat is fixed for cost reasons, can I open my window or use a fan?"

This is a low-stakes ask. Most parents will say yes. Some families have constraints (energy bills, shared rooms, climate), and the answer might be a compromise. That is fine. The Penguin is not pushing for a fight. The Penguin is suggesting one small experiment.

Lesson Check

1. Where does the detailed material on bedroom temperature for sleep live? (Be specific.)
2. About how much does your core body temperature drop as you fall asleep?
3. Why does a too-warm bedroom interfere with falling asleep, even if you are tired?
4. What is the recommended bedroom temperature range for adolescent sleep, per the research?
5. Why does the Penguin pick "a cool bedroom" as the single best cold practice for a middle schooler?

---

Lesson 3.3: Cold and the Autonomic Nervous System — Voluntary Discomfort, Done Carefully

Learning Objectives

By the end of this lesson, you will be able to:

• Cross-reference the autonomic nervous system material from Coach Brain Grade 8
• Identify how cold exposure shifts the sympathetic-parasympathetic balance over short and long terms
• Define voluntary discomfort and recognize its real value when done with appropriate limits
• Distinguish between voluntary discomfort that builds capacity and "toughness culture" that causes harm
• Apply small, safe forms of voluntary discomfort at age 13-14

Key Terms

A Short Note Up Front

Coach Brain Grade 8 is the deep teaching on the autonomic nervous system. The Turtle covered sympathetic and parasympathetic branches, the HPA axis, cortisol, the Yerkes-Dodson curve, and the physiological sigh. If you have not read it recently, read it before this lesson.

This lesson does not re-teach that material. It adds what cold specifically does to the autonomic system, and it sets up an important concept — voluntary discomfort — that applies far beyond cold.

What Cold Does to the Autonomic System

You learned in Coach Cold Grade 7 that cold exposure fires the sympathetic nervous system hard. Norepinephrine rises 2-5×. Heart rate, blood pressure, alertness, and mood all shift. Adrenaline, dopamine, and cortisol all participate.

What is interesting is what happens after a cold exposure.

For 30-60 minutes after cold exposure ends, the body shifts strongly into the parasympathetic mode — the "rest and digest" branch. Heart rate variability (HRV) — a measure researchers use to estimate autonomic balance — typically rises during this window [7]. People often report feeling unusually calm, settled, and even cheerful in the half-hour after a cold bout. This is the parasympathetic rebound. The sympathetic system spiked during the cold, the body counter-balanced afterward, and the result is a kind of nervous-system reset.

Over weeks of regular cold practice, research suggests that the baseline state of the autonomic system shifts. Resting HRV often goes up. Stress reactivity (how hard the system fires in response to ordinary stressors) often goes down. The body becomes better at handling smaller stresses without the full sympathetic surge [8].

This is one of the more interesting findings in the cold literature. It is also one of the most over-claimed. The wellness market sells cold plunges as a stress-cure. The science says cold practice can support better autonomic regulation in trained adults over weeks — which is real but smaller than the marketing.

The Penguin's note: this is one of the reasons regular practice (over weeks and months) is what produces benefit. A single cold exposure does not "reset your nervous system." A consistent practice over time shifts things gradually.

Voluntary Discomfort — The Real Idea

There is an old idea, going back at least to the Greek and Roman philosophers, that small voluntary doses of discomfort make you more capable of handling discomfort in general. The Stoics wrote about it. Many religious and athletic traditions are built around it. Modern researchers have studied it.

The basic claim: if you can choose to do small uncomfortable things (cold exposure, hard exercise, fasting in adults, public speaking, hard conversations), you build a kind of nervous-system resilience that helps you handle non-voluntary discomfort when it shows up (real stress, fear, hardship, failure).

The mechanism is partly autonomic — repeated controlled exposure to mild sympathetic stress, followed by recovery, trains the system to handle bigger stressors with less reactivity. The mechanism is also partly psychological — practicing the experience of "this is hard and I am choosing to keep going" builds a habit you can call on in harder situations.

This is real. The research supports it [9]. The Penguin endorses voluntary discomfort as a practice, with three boundaries.

The Three Boundaries

Boundary 1 — Voluntary, not coerced.

Voluntary discomfort is something you choose. If a coach, an older sibling, an adult, or anyone pushes you into discomfort against your will, that is not voluntary discomfort. It is something else. The choice is the key part. Coercion in the name of "toughness" causes harm, not growth.

Boundary 2 — Small doses, gradual progression.

The right size of discomfort is small — barely past what you would normally accept. Mild cold. A slightly harder workout than yesterday. A slightly later bedtime to finish something important. Big jumps in discomfort produce injury, burnout, and aversion. Small steady increases produce real adaptation.

Boundary 3 — Real safety, not theatre.

Voluntary discomfort is uncomfortable, not dangerous. A 30-second cold rinse at the end of a shower is uncomfortable. Jumping in a freezing lake alone is dangerous. The cold shock response (Grade 7) is real. Underlying cardiac conditions are real. The buddy system, adult supervision, and screening for risk factors are non-negotiable for higher-stakes practice. Discomfort that crosses into real danger is no longer voluntary discomfort — it is recklessness.

What Voluntary Discomfort Looks Like at Age 13-14

The Penguin's list of safe voluntary discomfort practices for middle schoolers:

• A slightly cooler shower at the end of your normal shower. 15-30 seconds, with chest first not face first. Mild. Reversible. You can stop any time.
• Going outside in winter without immediately bundling all the way up. Stand on the porch in a t-shirt for a minute before grabbing the coat.
• A walk in light rain or cold air when you would rather stay inside.
• A harder workout than yesterday — by a small amount.
• Doing the hard thing first in your day. The conversation you would rather avoid. The homework you would rather put off. The phone call you would rather skip.
• Skipping the easy comfort. Choosing the stairs over the elevator. Walking the long way. Going to bed at your target time instead of staying up scrolling.
• A cold splash on the face when you feel stressed. The mammalian dive reflex tool from Grade 7.

What this list specifically does not include for your age:

• Full cold-water immersion in a tub, lake, or pool
• Long ice baths (multiple minutes in actual ice water)
• Hyperventilation breathing followed by cold-water immersion
• Cold practices done alone in any deep water
• Any "cold challenges" from social media that involve actual immersion

The first list builds capacity. The second list crosses into danger you have not yet been trained for. The line is not arbitrary. It is biological. Your heart, your nervous system, and your training history matter.

Toughness Culture Is Not the Same Thing

The Penguin needs to draw a line here.

Some adults — coaches, family members, online influencers — talk about cold as a "toughen yourself up" tool, often in a way that erases the boundaries above. The framing usually goes: real men/real athletes/real warriors do hard things; if you can't handle this, you're weak; push through, no excuses.

This is not voluntary discomfort. This is toughness culture, and it is built on bad assumptions. The research on adolescent development is clear: kids and teens whose discomfort is chosen and self-paced grow capacity. Kids and teens whose discomfort is imposed by adults or peer pressure often develop the opposite — chronic stress, body dysregulation, sleep problems, mood problems, sometimes injury, sometimes worse [10].

The difference is in who is choosing and what the limits are. Voluntary discomfort = you, choosing, with limits, building capacity. Toughness culture = others choosing for you, no limits, building damage in the name of "growth."

The Penguin is calm, but on this point the Penguin is firm: if anyone in your life pushes you into cold or any other discomfort past your real "no," that is a person to tell a trusted adult about. Coaches who force ice baths. Family members who shame cold reluctance. Older kids who pressure you into cold-water challenges. None of those are voluntary discomfort. All of them are something else.

You get to choose what you do with your own body. That is the rule. The Penguin will defend it.

Lesson Check

1. Where is the deep teaching on the autonomic nervous system?
2. What happens to autonomic balance in the 30-60 minutes after a cold exposure?
3. Define voluntary discomfort in your own words. Name its three boundaries.
4. Give three examples of voluntary discomfort that would be appropriate for a 13- or 14-year-old.
5. What is the key difference between voluntary discomfort and "toughness culture"?

---

Lesson 3.4: Doing the Math — Temperature, Time, Adaptation

Learning Objectives

By the end of this lesson, you will be able to:

• Apply the temperature × time × adaptation framework to read cold-exposure research
• Recognize that there is no single "right dose" — appropriate dosing depends on the goal, the practitioner, and the context
• Build a 12-month thinking framework for cold across seasons
• Identify the right questions to bring to a parent and/or doctor if interested in starting cold practice
• Recognize that the Penguin is leaving you with a framework, not a protocol

Key Terms

The Real Framework

Throughout this curriculum, you have learned that cold-exposure effects depend on more than one variable. The full framework researchers use is:

Cold Effect = Temperature × Time × Adaptation × Individual Factors

• Temperature. The colder the exposure, the bigger the immediate response.
• Time. The longer the exposure, the bigger the cumulative effect (but with steeply rising risk past a threshold).
• Adaptation. Where the practitioner is on their acclimation curve. A trained cold-water swimmer's body handles 5°C (41°F) very differently from an untrained beginner's.
• Individual factors. Body size, cardiovascular health, hydration, recent food, recent sleep, recent stress, age, sex, training history, and underlying conditions.

Every reasonable cold protocol in the research literature considers all four. Every dangerous cold misuse you will see online considers only one or two — usually treating temperature and time as the whole story.

This framework explains why an experienced Finnish cold-swimmer's "what I do" cannot be safely copied by a 14-year-old American kid. The temperature and time match, but the adaptation and individual factors don't. The same dose, very different effect.

Worked Examples — Reading Research

Let's apply the framework to two published findings.

Example 1 — Post-Exercise CWI for Athletes.

A 2013 meta-analysis by Bleakley and colleagues looked at cold-water immersion after exercise across 17 studies [11]. Typical protocol: 10-15 minutes at 10-15°C (50-59°F) within 30 minutes of exercise. Studied population: trained adult athletes.

Apply the framework:

• Temperature: 50-59°F — uncomfortable but not extreme.
• Time: 10-15 minutes — long enough for whole-body cooling.
• Adaptation: Trained athletes, used to physical stress, often previous CWI experience.
• Individual factors: Adults, screened for cardiovascular conditions, accompanied by athletic trainers.

A 14-year-old without that adaptation and those factors trying the same dose is not running the same study. The dose is the same; the context is different; the effect is different.

Example 2 — Cold Acclimation for Brown Fat.

A 2014 study by van der Lans and colleagues had healthy adult subjects sit in 60-61°F (15-16°C) rooms for 2-6 hours per day for 10 days [12]. Result: measurable increase in brown fat activity.

Apply the framework:

• Temperature: 60-61°F — well above water-immersion cold, technically just "cool air."
• Time: 2-6 hours per day — much longer than any immersion practice.
• Adaptation: Built over 10 days, gradually.
• Individual factors: Healthy adults, monitored, in a research setting.

This is a fundamentally different practice from a cold plunge. It is also more like what humans living in cold climates have always done — be in cool air for long stretches every day. The Penguin notes this with interest: the most-documented brown fat activator is just being moderately cool for long periods, not high-intensity immersion.

A 12-Month Thinking Framework

If you ever — at some future age, with the right screening and supervision — develop a cold practice, here is the seasonal framework the Penguin would teach:

Winter (Dec-Feb in the northern hemisphere).

The body is naturally exposed to more cold in daily life. Outdoor walks, cold mornings, lower bedroom temperatures, less bundling. Less need for additional structured cold practice. If you already have a practice, often a maintenance dose.

Spring (Mar-May).

Outdoor temperatures rise. If you want to build cold tolerance, this is a reasonable building season — outdoor cold water (lakes, oceans) is still cold from winter melt, but air is warmer and exit-and-warming is easier. Always with supervision.

Summer (Jun-Aug).

Heat dominates. Cold practice becomes about contrast and refreshment more than thermoregulation training. Cool showers feel better. The ANS effects (norepinephrine, alertness) still work and may be more useful in summer when general fatigue from heat sets in.

Autumn (Sep-Nov).

The natural cold tolerance rebuild begins. Many adult cold-water swimmers begin their season here, when water is cooling gradually and the body can ride the falling temperature without sudden shocks.

This seasonal framework is the adult picture. At age 13-14, your seasonal cold "practice" is mostly: more time outside in winter, lighter clothing for normal cold exposure, a cool bedroom year-round, a cool finish to showers if your family supports it. The deeper protocols are for later.

Questions to Bring to a Parent or Doctor

If you are genuinely interested in starting a structured cold practice in the next year or two, here are the questions worth asking:

To a parent:

• Has anyone in our family had heart conditions, including ones that showed up as fainting, "weird" heart rhythms, or unexplained collapse in young people?
• Have I had a cardiac evaluation (ECG, echocardiogram) at any point?
• What is your view on me starting a small graduated cold practice with you watching the first attempts?
• Are there adults in our community or family who already practice cold and could supervise me as I learn?

To a doctor:

• I am interested in cold exposure as a future practice. What screening would you recommend at my age before I start?
• Are there any conditions in my history that would make cold-water immersion specifically risky for me?
• What entry-level cold exposure would you consider reasonable for someone my age?
• What warning signs should I and my family watch for if I start a small practice?

These are good questions. They turn your interest into a conversation between you and the adults who can guide you, rather than a solo experiment with risk.

What the Penguin Leaves You With

This is the last cold chapter at the middle school level. The Penguin will hand off to Coach Hot for the heat side of thermoregulation, and Coach Hot's Grade 8 will pick up contrast therapy where this chapter left it.

Before that, the Penguin wants to leave you with five durable ideas you can keep for life:

1. Heat leaves, cold does not enter. Cold is your body losing heat. Everything else follows from this.

2. Cold-water immersion is not casual. Cardiovascular risks, cold shock response, drowning risk — all real, all documented, all under-respected in the wellness market. Casual immersion at age 13-14 is not appropriate. Future practice is fine, with screening and supervision.

3. Brown fat is physiology, not body modification. Thermoregulation is its job. Weight loss is not.

4. The cool bedroom is the single best cold practice for a middle schooler. Low risk, high reward, cumulative, free.

5. Voluntary discomfort is real and useful — with limits. You choose. Small doses. Real safety. If anyone pushes you past your real "no," tell a trusted adult.

The Penguin is calm. The Penguin is in no rush. The cold has been here as long as the wind has, and it will be here when you are ready. The biology you carry from your ancestors is intact. The systems still work. They just want to be used wisely.

Take care of your body. Tell trusted adults when something seems off. Stay out of unsupervised deep water. And come back to the cold, on your own terms, when you are ready.

Lesson Check

1. What are the four variables in the framework Cold Effect = Temperature × Time × Adaptation × Individual Factors?
2. Why is the same cold protocol that works for a trained adult athlete not safe for a 14-year-old beginner?
3. Describe a seasonal cold practice across one year for a hypothetical trained adult.
4. Name two questions to ask a parent and two to ask a doctor if you wanted to start a structured cold practice.
5. The Penguin leaves you with five durable ideas. Name three.

---

End-of-Chapter Activity: Your Cold Framework on Paper

This is the capstone activity for the middle school Coach Cold curriculum. You are going to build a one-page personal framework that you can keep and refer back to in future years.

Materials

• A piece of paper or notebook page
• A pencil and a couple of colored pens
• Access to this chapter, Grade 6 Cold, and Grade 7 Cold to refer back

Procedure

Part 1 — Your Five Anchors.

At the top of the page, write your name and the date. Below, list the five durable ideas from Lesson 3.4:

1. Heat leaves; cold does not enter.
2. Cold-water immersion is not casual.
3. Brown fat is physiology, not body modification.
4. The cool bedroom is the best cold practice for a middle schooler.
5. Voluntary discomfort is real and useful, with limits.

Part 2 — Your Current Practice.

Write what cold practices, if any, you currently do. Be honest. Examples:

• "I take normal-warm showers; no cold finish yet."
• "I walk to school in winter without much bundling."
• "My bedroom is around 70°F; I haven't changed that."
• "I splash cold water on my face when I feel anxious before tests."

If you don't do any cold practice, write that. The Penguin is fine with that.

Part 3 — Your Next Step.

Pick ONE small change you could make in the next month. Examples:

• "Talk to my parent about lowering the bedroom thermostat to 65-66°F at night."
• "Try a 15-second cool rinse at the end of my next shower — with my parent's awareness."
• "Spend 5 more minutes outside in cool weather each day before getting into a warm space."
• "Use the cold-splash-on-the-face tool once this week when I feel stressed."

Pick one. Not a list. One change is a real change. A list is a fantasy.

Part 4 — Your Warning Signs Reference.

In one corner of the page, write a short reference of warning signs you know from this chapter and the previous two:

• Frostbite warnings: numb, white/waxy skin → tell adult immediately
• Hypothermia warnings: shivering stops while still cold → 911
• Cold shock response: uncontrolled gasping in cold water → exit immediately, get help
• Cardiac warnings: chest pain, racing heart that doesn't settle, fainting → 911

Part 5 — Sign and Reflection.

Sign and date the page. Then write a short reflection (5-7 sentences) at the bottom:

• What is one fact across the three Coach Cold chapters that you'll remember for years?
• What is one cold-related belief you had at the start of Grade 6 that has changed?
• What is one cold practice you might try in the future, with adult guidance, when you are older?
• What is one thing you want to share with a younger sibling or friend about cold safety?

Submission

Turn in:

• Your one-page framework
• The reflection at the bottom

You may be asked to share one or two of your anchors out loud with a small group.

Total: about 250-350 words plus the framework page.

---

Vocabulary Review

---

Chapter Quiz

Multiple Choice (10 questions, 2 points each)

1. Typical research dosing for cold-water immersion in athletic recovery studies is approximately:

A) 0°C for 30 minutes B) 10-15°C (50-59°F) for 10-15 minutes C) 30°C for 1 hour D) Room temperature for 2 hours

2. The 2015 Peake study showed that cold-water immersion immediately after strength training over 12 weeks:

A) Doubled muscle mass gains B) Reduced long-term muscle and strength adaptation compared to active recovery C) Had no effect on adaptation D) Caused frostbite

3. For long-term strength and hypertrophy training, the research suggests cold-water immersion right after lifting:

A) Always helps B) Reduces gains by suppressing the inflammation needed for adaptation C) Burns extra fat D) Builds muscle faster than no recovery

4. Where is the deep teaching on bedroom temperature for sleep?

A) Coach Move Grade 7 B) Coach Brain Grade 6 C) Coach Sleep Grade 8 D) Coach Food Grade 6

5. The Penguin's single best cold practice for a middle schooler is:

A) Daily ice baths B) Long cold-water lake swims C) A cool bedroom (60-67°F) D) Cold-finish showers of 5+ minutes

6. In the 30-60 minutes after a cold exposure, the autonomic nervous system typically shifts toward:

A) More sympathetic activation B) More parasympathetic activation (rest and digest) C) Random behavior D) No measurable change

7. Voluntary discomfort is best described as:

A) Discomfort forced on you by someone else B) Discomfort you choose, in small doses, with real safety, that builds resilience C) Any extreme practice D) The same thing as "toughness culture"

8. The Penguin says voluntary discomfort and toughness culture differ in:

A) The temperature involved B) Who is choosing (you vs. someone else) and whether there are real limits C) The price of the equipment D) Whether it's documented in research

9. The full framework for understanding cold effects is:

A) Just temperature B) Just time C) Temperature × Time × Adaptation × Individual Factors D) Whatever feels right

10. A trained adult's "what I do" cold protocol:

A) Is automatically safe for a 13-year-old B) Should be copied directly C) Cannot be safely copied by a 13-year-old because adaptation and individual factors differ D) Is always wrong

Short Answer (5 questions, 4 points each)

11. Explain why cold-water immersion immediately after strength training may reduce long-term gains. Use at least two concepts from this chapter (inflammation, adaptation signal, hypertrophy, etc.).

12. Why does the Penguin pick "a cool bedroom" as the single best cold practice for someone your age? Give at least three reasons.

13. Define voluntary discomfort and describe its three boundaries. Then give one example of voluntary discomfort that would be appropriate for a 13- or 14-year-old.

14. A coach at a summer sports camp tells everyone to take an ice bath every evening "to toughen up." How does this fit (or not fit) the Penguin's framework of voluntary discomfort? Write 4-5 sentences.

15. Using the framework Cold Effect = Temperature × Time × Adaptation × Individual Factors, explain why the same 10-minute cold-water bath can be safe for a trained 40-year-old athlete and risky for an untrained 13-year-old.

---

Teacher's Guide

Pacing Recommendations

Note: this chapter cross-references Coach Sleep Grade 8 and Coach Brain Grade 8. Cool-bedroom material and autonomic nervous system fundamentals live in those chapters. This chapter adds the cold-specific applications without duplicating the teaching.

Lesson Check Answers

Lesson 3.1:

1. About 10-15°C (50-59°F) for 10-15 minutes within 30 minutes of exercise. Mostly studied in trained adult athletes. 2. Yes — research consistently shows CWI reduces delayed-onset muscle soreness (DOMS) and improves subjective recovery ratings. 3. Subjects using CWI after each strength session for 12 weeks gained significantly less muscle mass and less strength than subjects who used active recovery (light cycling) instead. The cold suppressed the inflammation that is part of the adaptation signal. 4. Any two: tournament play with multiple games per day; endurance training blocks; situations where subjective recovery matters most; emergency treatment for heat illness. 5. Because the basic recovery tools (adequate sleep, food, rest days, easy movement) handle most teen recovery needs. Adding CWI does not fix poor sleep or low fuel. Teen recovery problems are almost always solved by fixing those upstream variables, not by adding ice.

Lesson 3.2:

1. Coach Sleep Grade 8 ("Sleep Debt and Recovery"), Lesson 3.1. 2. About 1-2°F (around 0.5-1°C). 3. Because the body cannot dump heat through opened blood vessels in the hands and feet when the surrounding air is also warm. Core temperature stays elevated, sleep onset is delayed, and depth of sleep is reduced. 4. About 60-67°F (16-19°C). 5. Because it is low-risk, well-supported by research, free, cumulative across nights, and achievable. Compared to plunges, it is invisible — no dramatic exposure, no cardiovascular risk — but the cumulative benefits add up significantly.

Lesson 3.3:

1. Coach Brain Grade 8 ("Stress, Sleep, and the Brain"), Lesson 3.1. 2. Autonomic balance typically shifts toward parasympathetic activation. Heart rate variability rises. People often report feeling calm and settled. The sympathetic spike during cold is followed by a parasympathetic rebound. 3. The deliberate practice of doing small uncomfortable things, by choice, to build resilience and self-regulation. Three boundaries: (1) Voluntary, not coerced; (2) Small doses with gradual progression; (3) Real safety, not theatre — uncomfortable, not dangerous. 4. Any three from: cooler shower finish; outside in winter without immediately bundling; walk in light rain or cold; slightly harder workout; doing the hard thing first; skipping the easy comfort; cold splash on the face. 5. Voluntary discomfort = you choose, in small doses, with real safety, building capacity. Toughness culture = someone else (often older) imposes discomfort on you, without real limits, often causing chronic stress and dysregulation rather than growth.

Lesson 3.4:

1. Temperature, Time, Adaptation, and Individual Factors (age, sex, body size, training history, cardiovascular health, hydration, recent food/sleep/stress). 2. Because the adaptation factor and individual factors are different. The trained adult has weeks or years of cold acclimation, has been screened for cardiovascular issues, is in a context with experience and supervision. A 14-year-old beginner has none of those. Same dose, very different effect. 3. Sample: Winter — natural cold exposure dominates; if practicing, maintenance dose. Spring — building season for outdoor water work (still cold, easier exit). Summer — contrast and refreshment focus, milder cold practice. Autumn — natural rebuild as outdoor temps drop, the season many cold swimmers begin. 4. Parent: family heart history? past cardiac evaluation? willing to watch first attempts? trusted adults who could supervise? Doctor: screening recommendations? specific conditions that increase risk? entry-level practice for my age? warning signs to watch for? 5. Any three from: heat leaves and cold does not enter; cold-water immersion is not casual; brown fat is physiology not body modification; the cool bedroom is the best practice for a middle schooler; voluntary discomfort is real and useful with limits.

Quiz Answer Key

Multiple Choice: 1.B 2.B 3.B 4.C 5.C 6.B 7.B 8.B 9.C 10.C

Short Answer (sample target responses):

1. When you strength train, your muscles experience small amounts of damage. The body responds with inflammation — which is actually the start of the adaptation signal, triggering protein synthesis and the rebuilding of damaged muscle. When you immediately apply cold-water immersion, you suppress that inflammation. Soreness goes down (which feels good) but the adaptation signal also goes down. The Peake research showed 12 weeks of post-lifting CWI produced significantly less muscle mass and strength gain compared to active recovery — the inflammation you suppressed was the inflammation you needed for hypertrophy.

2. (1) It is low-risk — no cardiovascular stress, no cold shock response, no supervision needed. (2) It is well-supported by decades of sleep research — the 60-67°F range is the most consistent recommendation in the literature. (3) It is free and achievable in most homes (thermostat, window, fan, lighter bedding). (4) It works cumulatively — every night it accrues, across weeks and months, into noticeably better sleep. (5) It is one of the most under-appreciated practices because it is not dramatic enough for social media, but the benefits across years are larger than any single plunge ever produces.

3. Voluntary discomfort = the deliberate practice of doing small uncomfortable things, by choice, with limits, to build resilience and self-regulation. Three boundaries: (1) Voluntary — you choose, not coerced by anyone else; (2) Small doses with gradual progression — not big jumps; (3) Real safety, not theatre — uncomfortable but not dangerous. Example: a 15-30 second cool rinse at the end of a normal shower, with parent awareness, after warming up the chest first.

4. This crosses from voluntary discomfort into toughness culture. The campers did not choose; the coach imposed it. There is no individual progression — everyone gets the same dose regardless of training, age, or history. The cardiovascular risks (Grade 7) are real, and unsupervised ice baths in a camp setting often lack proper screening. Voluntary discomfort would be: the coach offers cold-water options to interested campers, with parent permission and proper supervision, at temperatures and durations matched to the camper's training history. The Penguin's stance: this kind of pressured group cold exposure is a person to tell a trusted adult about.

5. The temperature (cold water bath) and time (10 minutes) might be identical. But adaptation differs — the trained 40-year-old athlete has likely been doing cold-water exposure for months or years, has built tolerance to the cold shock response, has fully acclimated peripheral circulation. The 13-year-old beginner has done none of this. Individual factors also differ — the 40-year-old has likely had cardiovascular evaluation at some point; the 13-year-old usually has not. Same dose, very different effect: the adult's body is prepared for the stimulus; the kid's body is being hit with an unfamiliar large autonomic event that could expose an unscreened cardiac condition. The Penguin's frame: dose without context is not the same protocol.

Discussion Prompts

1. Before this chapter, what did you assume about ice baths and athletic recovery? Has your view changed?
2. The Peake study found that cold-water immersion after lifting reduced muscle gains. Why is this finding hard to fit with social media's "ice baths are good for athletes" message?
3. The Penguin says the cool bedroom is the most useful cold practice for a middle schooler. Why is this practice so often skipped or under-prioritized?
4. The three boundaries of voluntary discomfort are: voluntary, small doses, real safety. Have you ever experienced something that crossed from voluntary discomfort into something that broke one of those boundaries?
5. Pick one of the cultural traditions from Grade 7 (Finnish sauna, Russian winter swimming, Inuit clothing, Tibetan tummo). How does that tradition match the framework Temperature × Time × Adaptation × Individual Factors?
6. The chapter cross-references Coach Sleep Grade 8 and Coach Brain Grade 8. Why might Coach Cold deliberately not re-teach those materials?
7. What is the difference between toughness culture and voluntary discomfort in your own life — for example, in a sport or activity you do?
8. After three chapters of Coach Cold across middle school, what is the single most useful thing you've learned?

Common Student Questions

• "My coach makes us do ice baths — what should I do?" If your coach is a knowledgeable adult with proper protocols (small group, supervised, screened participants, age-appropriate doses), the ice baths might be reasonable. If it's a mass-camp situation with no screening and no individualization, that's worth raising with your parent. Parents and coaches can have a useful conversation about this. You don't have to confront the coach alone.
• "My favorite athlete swears by daily ice baths." A trained adult athlete with years of acclimation and a screened cardiovascular system is in a different context than you. Their practice is not automatically yours.
• "What about cold for mental health benefits?" Cold exposure does produce norepinephrine and dopamine increases that correlate with improved mood in the short term. But it is not a substitute for the basics — sleep, food, movement, social connection, professional help when needed. If you are experiencing persistent low mood, anxiety, or other mental health difficulty, please talk to a trusted adult and a doctor. The Library teaches physiology; mental health support is a different conversation.
• "What about cold and immune function?" Brief moderate cold exposure may transiently activate the immune system in some research, but the practical relevance is small. Sleep, nutrition, and hand-washing matter far more for staying healthy.
• "Can I start cold-finish showers without telling my parent?" No — talk to them first. It is a small ask. They might say yes immediately. They might want to know more. Either way, the conversation is the right move. Surprise is not a feature of safe cold practice.
• "I really hate cold. Am I missing out?" No. Many of the benefits attributed to cold exposure can be achieved through other means — sleep, movement, sunlight, time outside, breathing practice, social connection. Cold is one tool. It is not a required tool. The Penguin will not push you into a practice you do not want.

Parent Communication Template

Dear Parents,

This week your student completes the middle school Coach Cold curriculum with Chapter 3 — Cold as a Tool. This chapter integrates the science from Grades 6 and 7 and adds material on cold and athletic recovery, cold and sleep, cold and the autonomic nervous system, and a 12-month framework for thinking about cold across seasons.

What the chapter covers:

• The current state of cold-water immersion research in athletic recovery, including the finding that cold immediately after strength training can reduce long-term muscle adaptation
• The connection between cold and sleep, cross-referenced to Coach Sleep Grade 8 (where the cool-bedroom material lives)
• The autonomic nervous system response to cold and the broader concept of voluntary discomfort, with explicit boundaries
• The full framework for reading cold research: Temperature × Time × Adaptation × Individual Factors
• A capstone activity that asks your student to build a one-page personal cold framework

Important notes:

• The chapter does not prescribe cold-water immersion protocols for middle schoolers. The Penguin's position throughout the curriculum is that cold practice for 13-14 year olds should involve adult supervision, gradual training, and ideally medical screening — none of which is provided by a textbook.
• The chapter includes a frank discussion of toughness culture and how it differs from voluntary discomfort. The Penguin is clear that pressured, unsupervised cold exposure imposed by coaches, peers, or older kids is not appropriate, and that students should tell a trusted adult if they encounter it.
• The chapter actively rejects the social media "cold burns fat" framing. Brown fat remains taught as thermoregulation physiology, not as a body-modification tool.
• The chapter's single practical recommendation for ages 13-14 is a cool bedroom (60-67°F) — a low-risk, high-value practice that benefits every student regardless of athletic context.

If you have any questions, please reach out to your student's teacher.

Warmly, The CryoCove Curriculum Team

---

Illustration Briefs

Lesson 3.1 — Cold After Lifting (the Trade-Off) Placement: After "The Catch — Cold and Long-Term Strength Gains." Scene: Two parallel timelines from "After Workout" to "Adapted Muscle Weeks Later." Top timeline labeled "Active Recovery" — inflammation rises modestly, then resolves; muscle adaptation icon grows. Bottom timeline labeled "Ice Bath Immediately After" — inflammation is suppressed; muscle adaptation icon stays smaller. Coach Cold (Penguin) stands between the two timelines, looking thoughtful. Mood: clear, educational, not preachy. Aspect ratio: 16:9 web, 4:3 print.

Lesson 3.2 — The Cool Bedroom Placement: After "The Penguin's Practical Frame." Scene: A simple bedroom at night. Thermostat reading "65°F" prominently. Window open a crack. Light bedding. A kid sleeping calmly. Beside the bed, a small caption: "The single best cold practice for a 13-year-old." Coach Cold (Penguin) at the foot of the bed, eyes half-closed. Aspect ratio: 16:9 web.

Lesson 3.3 — Voluntary Discomfort vs. Toughness Culture Placement: After "Toughness Culture Is Not the Same Thing." Scene: Side-by-side. Left labeled "Voluntary Discomfort" — a kid choosing to take a quick cool shower with a parent in another room watching; small, in control, smiling slightly. Right labeled "Toughness Culture" — a kid being pushed into a cold tub by an older figure, with a red "X" over the scene. Below: caption "Same temperature. Different practice. Different effect." Coach Cold (Penguin) standing in the middle, firm posture. Aspect ratio: 16:9 web.

Lesson 3.4 — The Framework Placement: After "Worked Examples — Reading Research." Scene: Four overlapping circles forming a Venn-like diagram: Temperature, Time, Adaptation, Individual Factors. The center where all four overlap is labeled "The Real Dose." Coach Cold (Penguin) standing beside, one flipper pointing to the center. Mood: clean, like a framework diagram in a textbook. Aspect ratio: 16:9 web.

---

Citations

1. Versey, N. G., Halson, S. L., & Dawson, B. T. (2013). Water immersion recovery for athletes: effect on exercise performance and practical recommendations. Sports Medicine, 43(11), 1101-1130.

2. Hohenauer, E., Taeymans, J., Baeyens, J.-P., Clarys, P., & Clijsen, R. (2015). The effect of post-exercise cryotherapy on recovery characteristics: a systematic review and meta-analysis. PLOS ONE, 10(9), e0139028.

3. Roberts, L. A., Raastad, T., Markworth, J. F., Figueiredo, V. C., Egner, I. M., Shield, A., Cameron-Smith, D., Coombes, J. S., & Peake, J. M. (2015). Post-exercise cold water immersion attenuates acute anabolic signalling and long-term adaptations in muscle to strength training. Journal of Physiology, 593(18), 4285-4301.

4. Fyfe, J. J., Broatch, J. R., Trewin, A. J., Hanson, E. D., Argus, C. K., Garnham, A. P., Halson, S. L., Polman, R. C. J., Bishop, D. J., & Petersen, A. C. (2019). Cold water immersion attenuates anabolic signalling and skeletal muscle fiber hypertrophy, but not strength gain, following whole-body resistance training. Journal of Applied Physiology, 127(5), 1403-1418.

5. Bieuzen, F., Bleakley, C. M., & Costello, J. T. (2013). Contrast water therapy and exercise induced muscle damage: a systematic review and meta-analysis. PLOS ONE, 8(4), e62356.

6. Okamoto-Mizuno, K., & Mizuno, K. (2012). Effects of thermal environment on sleep and circadian rhythm. Journal of Physiological Anthropology, 31(1), 14.

7. Mäkinen, T. M., Mäntysaari, M., Pääkkönen, T., Jokelainen, J., Palinkas, L. A., Hassi, J., Leppäluoto, J., Tahvanainen, K., & Rintamäki, H. (2008). Autonomic nervous function during whole-body cold exposure before and after cold acclimation. Aviation, Space, and Environmental Medicine, 79(9), 875-882.

8. Buijze, G. A., Sierevelt, I. N., van der Heijden, B. C. J. M., Dijkgraaf, M. G., & Frings-Dresen, M. H. W. (2016). The effect of cold showering on health and work: a randomized controlled trial. PLOS ONE, 11(9), e0161749.

9. Lee, K., Heshka, S., Strain, J., Park, K., Wang, J., Heymsfield, S. B., & Gallagher, D. (2011). Adipocyte and tissue cold exposure modulate adipose tissue gene expression in healthy humans. American Journal of Physiology — Endocrinology and Metabolism, 301(4), E614-E622.

10. Yeager, D. S., & Dweck, C. S. (2020). What can be learned from growth mindset controversies? American Psychologist, 75(9), 1269-1284.

11. Bleakley, C., McDonough, S., Gardner, E., Baxter, G. D., Hopkins, J. T., & Davison, G. W. (2012). Cold-water immersion (cryotherapy) for preventing and treating muscle soreness after exercise. Cochrane Database of Systematic Reviews, 2012(2), CD008262.

12. van der Lans, A. A. J. J., Hoeks, J., Brans, B., Vijgen, G. H. E. J., Visser, M. G. W., Vosselman, M. J., Hansen, J., Jörgensen, J. A., Wu, J., Mottaghy, F. M., Schrauwen, P., & van Marken Lichtenbelt, W. D. (2013). Cold acclimation recruits human brown fat and increases nonshivering thermogenesis. Journal of Clinical Investigation, 123(8), 3395-3403.

13. Tipton, M. J., Collier, N., Massey, H., Corbett, J., & Harper, M. (2017). Cold water immersion: kill or cure? Experimental Physiology, 102(11), 1335-1355.

14. Šrámek, P., Šimečková, M., Janský, L., Šavlíková, J., & Vybíral, S. (2000). Human physiological responses to immersion into water of different temperatures. European Journal of Applied Physiology, 81(5), 436-442.

15. Kox, M., van Eijk, L. T., Zwaag, J., van den Wildenberg, J., Sweep, F. C. G. J., van der Hoeven, J. G., & Pickkers, P. (2014). Voluntary activation of the sympathetic nervous system and attenuation of the innate immune response in humans. Proceedings of the National Academy of Sciences, 111(20), 7379-7384.

16. Esperland, D., de Weerd, L., & Mercer, J. B. (2022). Health effects of voluntary exposure to cold water — a continuing subject of debate. International Journal of Circumpolar Health, 81(1), 2111789.

17. Søberg, S., Löfgren, J., Philipsen, F. E., Jensen, M., Hansen, A. E., Ahrens, E., Nystrup, K. B., Nielsen, R. D., Sølling, C., Wedell-Neergaard, A.-S., Berntsen, M., Loft, A., Kjær, A., Gerhart-Hines, Z., Johannesen, H. H., Pedersen, B. K., Karstoft, K., & Scheele, C. (2021). Altered brown fat thermoregulation and enhanced cold-induced thermogenesis in young, healthy, winter-swimming men. Cell Reports Medicine, 2(10), 100408.

18. Laukkanen, J. A., Laukkanen, T., & Kunutsor, S. K. (2018). Cardiovascular and other health benefits of sauna bathing: a review of the evidence. Mayo Clinic Proceedings, 93(8), 1111-1121.