Chapter 3: Heat as System

Chapter Introduction

You have learned what heat does inside a single body, and you have learned what humans have done with heat across centuries of practice. This chapter asks the question that the body itself has been asking the whole time:

How does heat sit in conversation with everything else?

Heat is not a separate room. Your body is a system, and any practice you bring to that system — heat, cold, movement, sleep, breath, food, light, water, the things you think about — ripples through every other practice. A useful sauna session at the wrong time can make sleep harder. A useful workout in the heat without adequate preparation can leave you depleted for days. A long history of patient heat practice in an adult population can show up decades later as a different cardiovascular profile. None of this is magic. It is physiology, scaled across hours, days, decades.

This chapter is built around four ways heat lives in conversation with the rest of the body's systems:

In the first lesson, you will study heat and exercise — how heat changes what training does, why athletes use heat strategically, and where the line is between heat-as-training-tool and heat-as-burden-on-performance. In the second, heat and sleep — why sleep is one of the most heat-sensitive things you do, why nighttime cooling is part of normal physiology, and why morning or afternoon heat is often easier to integrate than evening heat. In the third, heat and the cardiovascular system across time — what research has observed about repeated heat exposure and vascular function, and what mechanisms are believed to underlie the long-term changes. In the fourth, heat and longevity — the careful, descriptive look at what observational research in adult populations has suggested about heat practice and human aging, what the mechanisms might be, and how a junior in high school should think about results from research in their grandparents' generation.

This is system-level thinking. The Camel asks you to look up from the immediate practice and see the long view. Not because you have to plan your seventh decade today — but because every habit you build at 16 plants something. Some habits compound across decades. Heat practice, if it becomes part of your life, is one of those.

A note on the limits of this chapter. The research base described here was overwhelmingly done in adult populations — middle-aged adults, older adults, athletes in their adult years. The Camel describes that research carefully and refuses to translate it into protocols for teenagers. Where the chapter touches sleep, exercise, and cardiovascular adaptation, you will see general principles that apply to growing bodies — drawn from the broader sleep, exercise, and cardiovascular literatures — and descriptive accounts of what research has observed in adults. Specific heat protocols belong to you, your family, and your healthcare provider together.

Now: the long view.

---

Lesson 3.1: Heat and Exercise

Learning Objectives

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

• Describe how exercise in heat changes cardiovascular and metabolic demand compared with exercise in cool conditions
• Identify what "heat as training stimulus" means for adult athletes
• Distinguish between performance in heat and adaptation to heat
• Recognize the warning signs of overheating during exercise and the role of pacing
• Apply the principle that heat is an additive stress on top of training stress

Key Terms

Exercise Already Generates Heat

Before we look at exercise in heat, recognize this: exercise generates heat. Skeletal muscle is one of the most thermogenic tissues in your body. When you run, lift, swim, climb, or play, your muscles convert chemical energy into mechanical work, and a substantial fraction — typically 70-80 percent of the energy used — leaves as heat [1]. You are an internal furnace any time you move hard.

This is why core temperature rises during exercise even in cool conditions. A marathon runner's core temperature can climb to 39°C (102°F) during a fast race. Endurance athletes routinely tolerate core temperatures that would be a fever in another context. The body has worked out, across hundreds of thousands of years of human movement, how to tolerate this kind of internal heat generation.

The challenge of exercising in environmental heat is that you are now asking your cooling systems to handle both the heat you make from inside and the heat coming from outside. The system has the same cooling capacity. The demand has roughly doubled. Something has to give [2].

Thermal Strain and Cardiovascular Drift

When you run a 10-kilometer pace on a cool 12°C autumn day, your heart rate, breathing, and core temperature reach a steady-state within a few minutes and stay relatively stable.

When you run the same 10-kilometer pace on a hot 32°C summer day, your heart rate climbs from a steady state. You may start at 150 beats per minute and find yourself, twenty minutes in, at 165 beats per minute at the same pace. This is cardiovascular drift — the gradual rise in heart rate during sustained exercise in heat [3]. The system is having to work harder over time to manage the heat. Skin blood flow expands. Plasma volume drifts as sweat continues. The cardiovascular system is doing more total work than it would in cool conditions.

The practical implications:

• The same training run feels harder in heat.
• A "tempo run" pace in heat is, in cardiovascular terms, a higher-intensity effort than the same pace in cool conditions.
• Recovery from a hot-condition training session is often longer than from an equivalent cool-condition session.
• Pacing must adjust. Athletes who try to run their cool-condition paces in heat tend to either slow involuntarily later in the workout or accumulate severe thermal strain.

A reasonable rule among endurance coaches: at temperatures above about 25°C and significant humidity, slow your pace by 5-10 percent. At above 30°C with high humidity, slow further. This is not weakness. It is honest physiology [4].

Heat as Training Stimulus (in Adults)

Here is where heat becomes more than just a burden on training. Several lines of research in adult athletic populations have observed that the same adaptations that develop during heat acclimatization — plasma volume expansion, improved cardiovascular function — appear to carry over into improved performance in cool conditions [5].

This is the basis for "heat acclimation training" in adult endurance athletes. Some adult athletes deliberately train in hot environments — or use saunas, hot tubs, or heated rooms after training — to provoke the adaptations of heat acclimatization. When those athletes return to cool-condition competition, their expanded plasma volume and improved cardiovascular function may produce measurable performance benefits. Several studies have documented this effect, though it varies by athlete, protocol, and sport [6].

A note: Coach Hot is not telling you to do this. Heat acclimation training is one of the most demanding things an adult endurance athlete can do, requires careful monitoring, and is appropriate only in the context of a coached training program with adult support. The teen takeaway is more modest: the cardiovascular adaptations the body makes to heat are real and meaningful. Even ordinary heat acclimatization — adapting comfortably to a hot summer in your home town — is doing some of the same work.

Cooling Strategies When Exercise Must Happen in Heat

Sometimes you cannot move the workout to a cooler time. A football practice is on the schedule. A cross-country race is in August. A summer hiking trip is what it is. When exercise must happen in heat, the question becomes: how do you manage thermal strain?

Researchers and athletic trainers have studied many cooling strategies, and several are well-supported [7]:

Pre-cooling. Cooling the body before exercise — cold beverages, ice-slurry drinks, cool environment in the warm-up — provides some additional thermal "reserve" before the workout starts. The cooler the body starts, the longer until thermal strain becomes performance-limiting.

Mid-cooling. Brief cooling during the activity — cold water sponging, ice towels at rest stops, cooling vests in sports that allow them, cool drinks — can lower skin temperature briefly and reduce subjective heat distress.

Post-cooling. Cooling after exercise speeds recovery and reduces lingering thermal strain. A cool shower, cool drinks, time in air-conditioned space.

Pacing. As discussed: starting at a sustainable pace, given the conditions, rather than the pace you would run in cool weather.

Hydration around the activity. As covered in Grade 10, hydration is a practice, not a reaction. Pre-hydration, during-activity hydration, and post-rehydration all matter.

Acclimatization. Over 7-14 days of exposure to hot conditions before a competition or sustained training period, the body adapts as described in Grade 10.

Clothing and equipment. Light-colored, breathable clothing reflects sun and allows sweat evaporation. Bulky uniforms or equipment increase thermal load substantially — one reason football preseason has particularly careful heat protocols.

None of these strategies prevent heat illness if conditions are extreme enough. The Camel reminds you again: pacing and respect for the conditions trump every cooling trick. There are days when the right answer is to move the workout indoors or to a cooler hour. Coaches and athletic trainers who do this are not soft; they are protecting you.

When to Stop

The signs of dangerous overheating during exercise are not subtle once you know them:

• Pace inexplicably slowing despite effort
• Heart rate climbing without performance change
• Confusion or dizziness
• Nausea or stomach distress beyond ordinary
• Skin that stops sweating despite hot conditions, or that becomes very flushed or very pale
• Inability to follow normal conversation
• A feeling of "the wheels coming off" that experienced athletes recognize

If you see these in yourself or a teammate, stop. Get cool. Get fluids. Get adult help if any sign points to heat exhaustion or worse. There is no race, practice, drill, or set worth pushing past these signals. A practice missed for heat is recoverable. Heat illness is sometimes not, and severe heatstroke can cause lasting damage or be fatal [8].

Coach Hot wants you to be the teammate who notices, who speaks up, who is willing to look like the "cautious one" if needed. The Camel does not walk into the worst heat of the day to prove something. The Camel walks at dawn. The Camel walks at dusk. The Camel rests at noon.

Lesson Check

1. Why does heart rate "drift" upward during prolonged exercise in heat even at constant pace?
2. By roughly what percentage do endurance coaches commonly recommend slowing pace in significant heat and humidity?
3. What does "heat as training stimulus" mean, and why is it used by some adult athletes?
4. List three practical cooling strategies that athletic programs use during exercise in heat.
5. List three warning signs of dangerous overheating during exercise.

---

Lesson 3.2: Heat and Sleep

Learning Objectives

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

• Describe the body's natural temperature drop during the sleep cycle
• Explain why a cool sleep environment supports better sleep
• Identify how evening heat exposure can interfere with sleep onset
• Recognize the principle that heat practice and sleep timing should be planned together
• Apply this knowledge to environmental, behavioral, and seasonal sleep planning

Key Terms

The Sleeping Body Is a Cooler Body

If you measure your body temperature continuously across a day, you find a clear pattern. Temperature rises gradually through the morning and reaches its peak in late afternoon or early evening — around 4-7 pm in many people. Then it begins to fall. By the time you are normally asleep, your core temperature has dropped by about 0.5-1°C from its daytime peak [9]. It continues falling through the early part of the night. Around 4-5 am, it reaches its lowest point, and then begins climbing back toward your waking temperature.

This pattern is built in. It is part of your circadian rhythm. Your body uses temperature change as one of the signals that prepares you for sleep, sustains sleep, and prepares you for waking. Sleep is not just a brain event. It is a whole-body thermal event.

This is why sleep is profoundly sensitive to environmental temperature. A bedroom that is too warm interferes with the body's ability to drop its core temperature. The hypothalamus, sensing that the environment will not let it cool, may keep skin blood flow patterns awake, delay sleep onset, fragment sleep through the night, or reduce the time spent in deep slow-wave sleep [10]. People who chronically sleep in warm rooms often report poorer sleep quality, more nighttime awakenings, and morning fatigue — even when their sleep duration looks adequate.

What the Sleep Researchers Recommend

Decades of sleep research have produced a fairly consistent recommendation for bedroom temperature in adults: somewhere in the range of 16-19°C (60-67°F) [11]. This is cooler than most people set their thermostats during the day. It is closer to the temperature your body is trying to drop toward.

For adolescents, the same general principle applies, though individual preferences vary. Some teenagers sleep better at 18°C; some prefer 20°C; some find anything below 19°C uncomfortable initially and acclimate over a few weeks. The principle is clear: a cooler bedroom supports better sleep than a warmer one.

This connects directly to seasonal experience. Sleep in winter — when the house is cool and the bedroom is allowed to stay cool — is often subjectively better than sleep in summer, when heat lingers. The difference is not random. The body is trying to do its nightly cooling, and the environment is either helping or fighting.

Practical implications:

• Keep the bedroom cooler than the rest of the house overnight.
• Use lighter bedding in warmer seasons; layer in cool seasons.
• A fan that moves air across your skin helps evaporate small amounts of moisture and feels cooler than still warm air.
• If the home cannot be cooled, a fan, a cooler window, a damp cloth on the wrists or neck before bed, and lighter clothing all help.
• Avoid eating large meals or vigorous exercise in the last 1-2 hours before sleep, both of which raise core temperature.

What Heat Exposure Does to Sleep

The question this lesson is built around: how does deliberate heat practice — a sauna, a hot bath, a hot workout — interact with sleep?

The answer turns out to depend heavily on timing.

Heat exposure in the morning or early afternoon generally does not interfere with that night's sleep. The body has many hours to cool back down and resume its normal temperature trajectory. Adult sauna users in Finland often practice in the late afternoon or early evening with sleep just fine, but the morning slot is the most sleep-neutral if scheduling allows.

Heat exposure in the late afternoon or early evening can cut both ways. The body cools back down over the next 1-3 hours. If the heat exposure is followed by adequate cool-down time before bed (often 90 minutes or more), some research has observed that the post-heat drop in core temperature actually deepens sleep onset — the body's cooling trajectory continues right into the sleep period [12]. A warm bath 90 minutes before bed is a sleep-hygiene technique sometimes recommended for this reason. The principle: heat that ends well before sleep can help.

Heat exposure immediately before sleep is more disruptive. If the body is still warm from a workout, sauna, or hot shower at the moment you are trying to fall asleep, core temperature has not yet dropped. Sleep onset is delayed. Slow-wave sleep is often reduced. This is one of the reasons high-intensity evening workouts can compromise sleep for some people, and one reason saunas are not generally taken in the last hour before bed in cultures with long traditions.

What This Means for a Student

You are a junior in high school. You have a complicated schedule. You have homework. You have athletic practice. You may have a job. You may want to figure out where heat practice fits.

The principles to apply, regardless of specific protocol:

1. Cool bedroom matters more than anything else you can do about heat and sleep. If you have control over your sleeping environment, make it cool.
2. Morning and early afternoon are sleep-friendly times for heat. A warm bath, a sauna with family, a hot summer workout — all generally have time to clear before bed.
3. A late-afternoon or early-evening heat exposure can support sleep if there is at least 90 minutes of cool-down before bed. The post-heat cooling can deepen sleep onset.
4. Heat in the last hour before bed is usually not ideal. Hot showers right before sleeping, intense workouts late at night, or evening saunas without recovery time tend to push sleep onset later and reduce sleep quality.

The Camel does not panic about timing. The Camel notices. If a practice helps your sleep, keep it. If a practice seems to be making sleep worse, move it earlier or change it. Your body is the data, and your body is honest if you listen.

Seasonal Heat and Sleep

A final note on seasons. Summer heat is one of the most common causes of chronic mild sleep disruption in adolescents. If your bedroom heats up during the day and stays warm into the evening, your nightly cooling is fighting upstream. Several practical strategies:

• Close blinds during the day to keep solar heat out
• Open windows in the cool evening and close them in the morning to capture cool night air
• Fans rather than AC if AC is not available (fans use far less energy and produce subjective cooling)
• If a bedroom is on a sunny side, even a few hours of evening cool-down before bedtime helps
• Lighter sheets, lighter pajamas, less layered bedding for the season

The Camel knows seasons. The Camel sleeps differently in winter than in summer, but the Camel always sleeps. Adjusting your environment to support nightly thermal cooling is one of the most underused sleep practices available.

Lesson Check

1. Describe what happens to core body temperature across a 24-hour period and how it relates to sleep.
2. What is the recommended bedroom temperature range commonly suggested by sleep research?
3. Why might a warm bath 90 minutes before bed actually help sleep, while a hot shower right before bed might hurt it?
4. List three practical environmental changes a student could make to support better summer sleep.
5. If heat practice is in your life, what time of day is most sleep-neutral, and why?

---

Lesson 3.3: Heat and the Cardiovascular System Across Time

Learning Objectives

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

• Identify the major cardiovascular adaptations that develop with repeated heat exposure over weeks to months
• Distinguish between acute responses (one session), training adaptations (weeks), and long-term changes (months to years)
• Describe what research has observed about heat exposure and arterial function in adult populations
• Explain mechanisms that researchers have proposed for these long-term changes
• Apply system-level thinking: how a small repeated stimulus accumulates into a meaningful change

Key Terms

Three Timescales

When you encounter heat — a single sauna, a hot summer day, a workout in warmth — your body has a series of immediate responses. We covered them in Chapters 1 and 2: skin blood flow rises, heart rate climbs, sweat begins, core temperature edges up slightly, then returns to baseline as you cool. This is the acute response.

When you encounter heat repeatedly over a week or two, your body builds adaptations: plasma volume rises, sweat onset comes earlier, sweat becomes more dilute, heart rate at given effort drops, cardiovascular efficiency improves. This is the training adaptation.

When you encounter heat consistently over months and years — as Finnish sauna users do, as many summer-climate workers do, as some adult endurance athletes deliberately do — research has observed a third layer of change: shifts in endothelial function, arterial stiffness, blood pressure, and other markers of long-term cardiovascular health. This is the long-term adaptation, and it is what makes the cardiovascular research on regular sauna use so interesting [13].

This lesson is about that third layer. Not because you are going to spend the next 30 years in a sauna — but because system-level thinking includes understanding how small repeated stimuli compound into meaningful change.

Endothelial Function: The Thin Layer That Matters Enormously

Every blood vessel in your body is lined on the inside with a thin layer of cells called the endothelium. Until about the 1980s, the endothelium was considered a passive barrier — a smooth surface for blood to flow over. We now know it is one of the most active and important tissues in the body. The endothelium produces signals (most famously nitric oxide) that cause blood vessels to relax and widen. It regulates inflammation. It influences blood clotting. It modulates blood flow moment by moment.

When endothelial function is healthy, vessels dilate appropriately in response to demand, blood pressure regulates smoothly, and the system is responsive. When endothelial function is impaired — by chronic high blood pressure, smoking, poor metabolic health, certain disease states, or aging — vessels become less responsive, blood pressure becomes harder to regulate, and risk of cardiovascular events rises [14].

Several lines of research in adult populations have observed that regular heat exposure improves measures of endothelial function. Studies using flow-mediated dilation as the measurement have observed improvements after weeks of repeated heat exposure in sedentary adults [15]. The mechanism appears to involve the repeated rise in shear stress (the force of blood flowing across the endothelium) during the skin blood flow expansion of heat exposure. The endothelium, exposed repeatedly to this gentle, rhythmic stress, builds capacity.

In simpler language: heat exposure asks your blood vessels to do more, repeatedly. Like a muscle responding to use, the vessels respond by becoming better at the job.

Arterial Stiffness: The Flexibility of Aging

Your large arteries — the aorta and its major branches — are not rigid pipes. They are elastic tubes that expand slightly with each heartbeat and recoil between beats. This elasticity has important consequences. Flexible arteries cushion the pulse of blood from the heart, distribute pressure smoothly, and reduce the workload on the heart. Stiffer arteries do not cushion as well, blood pressure swings more sharply, and the heart works harder.

Arterial stiffness tends to increase with age. It also increases with chronic high blood pressure, sedentary behavior, smoking, and certain disease states. It decreases with regular aerobic exercise. And — interestingly — it has been observed to decrease with periods of regular heat exposure in adult populations [16].

The mechanism is believed to involve some of the same pathways: improved endothelial function, repeated mild stress on the vessel walls that triggers maintenance and remodeling, possible reductions in chronic low-grade inflammation. Whether these effects accumulate across decades the way regular exercise effects do is still being studied, but the cohort data from Finnish populations is consistent with a long-term cardiovascular benefit in adults who use sauna frequently [13, 16].

Heat Shock Proteins: A Cellular Story

There is a deeper mechanism that may be part of what is going on. When cells are exposed to mild heat stress, they produce a family of proteins called heat shock proteins (HSPs). These proteins do several jobs:

• Refold proteins that have been mildly damaged by stress, returning them to functional shape
• Mark severely damaged proteins for degradation
• Protect cells from subsequent stress (the cell is "primed" for further challenge)
• Modulate inflammation responses

Heat shock proteins were first discovered in the 1960s in fruit flies that had been briefly heated, and have since been observed across nearly all forms of life [17]. In humans, repeated mild heat exposure increases baseline levels of heat shock proteins in tissues including muscle, vascular tissue, and others. Researchers have proposed that this elevated baseline contributes to cellular resilience — the body, having been repeatedly asked to handle mild heat stress, is better prepared for various forms of physiological challenge.

This is one specific example of the broader principle of hormesis — that a small, repeated stress produces adaptations that make a system stronger. Exercise is hormesis. Cold exposure is hormesis. Heat exposure is hormesis. So are many other practices the human body responds well to. The challenge is always finding the dose: small enough to be a productive stress, large enough to provoke adaptation, and not so large that it becomes harm.

What Researchers Mean When They Say "Heat as Cardiovascular Practice"

Putting these pieces together, you can understand why some adult research communities have begun describing regular heat exposure as a kind of "cardiovascular practice." The pieces include:

• Acute responses that overlap with moderate exercise (elevated heart rate, increased cardiac output, vasodilation)
• Training adaptations that overlap with endurance training (plasma volume expansion, improved heart rate efficiency)
• Long-term changes that overlap with the cardiovascular effects of regular exercise (improved endothelial function, reduced arterial stiffness)
• Cellular mechanisms including heat shock proteins that overlap with other forms of mild stress adaptation
• Population-level observations associating frequent sauna use with lower rates of cardiovascular events in adult cohorts

Coach Hot wants you to understand the appeal — and the limits — of this framing. The appeal: there is real biology underneath. The limits: heat is not exercise. Heat does not strengthen muscles, build bones, train coordination, or develop neuromuscular skill. The cardiovascular features are real but partial. A person who sat in a sauna for an hour every day but never moved would gain some cardiovascular benefit and miss the entire rest of what their body needs.

The integrated picture: regular movement is the cardiovascular foundation. Heat exposure, where it fits a life, adds a useful additional layer. Both have research support in adults. Neither replaces the other.

A Note for the Junior Reader

You are 16, 17 years old. Your cardiovascular system is in its developmental prime. Your endothelium is supple. Your arteries are flexible. Your heart is adapting to every load you put on it. You are, in cardiovascular terms, building the system you will live with for the next 70-80 years.

The most important cardiovascular practices at your age are not exotic. Move your body. Run. Lift. Climb. Swim. Walk. Play. Get adequate sleep. Eat real food. Manage chronic stress when you can. These ordinary practices, done consistently across your teens and twenties, build a cardiovascular system that ages well [18].

Heat practice can be part of this — at the margins, in a family context, when and how the adults in your life decide it fits. The research described in this lesson should not be read as a teen prescription. It should be read as an explanation: here is why people in their forties and fifties are taking heat practice seriously, here is what they have observed, here is what some of the mechanisms appear to be. The long view is being built now, in your habits, in your patterns, in the relationship with your body you are forming this year.

The Camel does not ask you to live like an old camel. The Camel asks you to be a thoughtful young one. The desert is long.

Lesson Check

1. What is the endothelium, and why is it important for cardiovascular health?
2. Describe arterial stiffness and how it tends to change across the lifespan.
3. What are heat shock proteins, and how do they relate to the concept of hormesis?
4. Why is heat exposure not a substitute for movement, even though it shares some cardiovascular features?
5. What are the most important cardiovascular practices at your age, regardless of whether heat practice is part of your life?

---

Lesson 3.4: Heat and Longevity

Learning Objectives

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

• Describe what observational longevity research in adult populations has suggested about regular heat exposure
• Distinguish between association and causation in long-term cohort studies
• Identify the limits of applying adult longevity data to teenage decision-making
• Recognize that "longevity" is not a single number but a set of qualities — health, function, resilience, quality of life
• Apply long-view thinking to your own habits without succumbing to "longevity optimization" framing

Key Terms

What the Finnish Sauna Data Actually Says

The most discussed longevity research on heat exposure comes from a long-running prospective cohort study in Eastern Finland, often abbreviated as the Kuopio Ischaemic Heart Disease Risk Factor Study (KIHD). This study has followed thousands of middle-aged men since the 1980s, tracking their health behaviors and various outcomes [19].

Key findings published from this dataset:

• Men who used the sauna 4-7 times per week had a 40 percent lower hazard ratio for all-cause mortality compared with men who used the sauna once per week (over a roughly 20-year follow-up) [19].
• Men with high sauna frequency had a 50 percent lower hazard ratio for fatal cardiovascular events [19].
• Longer single-session sauna duration (more than 19 minutes) was also associated with lower rates of fatal cardiovascular events [19].
• Frequent sauna use was associated with lower rates of dementia and Alzheimer's diagnoses in this cohort [20].
• Several other associations have been published, including reduced rates of certain pulmonary outcomes and some others.

These are striking numbers. They are also from one cohort study in one country, in a specific cultural context, with the specific limitations of observational research. Coach Hot wants you to understand both the strength and the limits.

Strengths and Limits of This Kind of Evidence

Strengths. The KIHD study is large (over 2,300 men), prospectively designed, long in follow-up (20+ years), and conducted in a population where sauna use is a deeply embedded cultural practice, which reduces (though does not eliminate) the social-selection issues that can confound similar studies in cultures where sauna users self-select more strongly. The associations are large enough that even after adjusting for many variables (smoking, exercise, body mass, diabetes, cholesterol, alcohol, etc.), they remain significant. The dose-response pattern (more sauna, larger effect) is consistent with a real biological signal rather than chance.

Limits. Observational cohort studies cannot prove that one behavior causes another outcome. Several questions remain:

• Confounding. Even after careful statistical adjustment, residual confounding is possible. Frequent sauna users may share other lifestyle features that the researchers could not fully measure — different social engagement, different stress patterns, different time outdoors. Some of the apparent benefit may belong to those features rather than to sauna itself.
• Generalizability. The KIHD cohort is middle-aged Finnish men. Whether the findings generalize fully to women, to other ethnic groups, to other ages, or to cultures with different sauna practices is not fully established. Some follow-up studies in other populations have replicated parts of the findings; some have produced different effect sizes.
• Reverse causation. Healthier people may use the sauna more because they feel better. Sicker people may use it less because heat is uncomfortable. This is a smaller concern in long-term prospective studies, but it never disappears entirely.
• No randomized trials at this scale. A randomized controlled trial of sauna use across 20 years would be virtually impossible to conduct. So the body of evidence for long-term outcomes will remain primarily observational.
• Cultural specificity. Sauna in Finland is a multigenerational, family, social, and sometimes religious-adjacent practice. Adopting the practice without that cultural scaffolding may not reproduce the same effects.

The honest summary: the Finnish data is consistent with regular heat exposure being part of a long, healthy adult life. It is not proof that adding sauna will extend your life. The mechanisms described in the previous lesson — endothelial function, plasma volume, heat shock proteins, autonomic balance — give the observational findings biological plausibility. The conclusion is best stated as: there is supportive but not definitive evidence that regular heat exposure across adult life is associated with various favorable outcomes.

Healthspan, Not Just Lifespan

The framing of "longevity" in popular media often emphasizes total years lived. The more useful framing is healthspan — years lived in good health, with function, with relationships, with the ability to do what you want to do [21].

Healthspan is shaped by many practices, not by any single one:

• Movement — strength, cardiovascular health, range of motion, balance
• Nutrition — adequate protein across the lifespan, micronutrient sufficiency, food relationship
• Sleep — consistent quality sleep across decades
• Social connection — relationships sustained across life stages
• Cognitive engagement — learning, curiosity, attention
• Stress regulation — capacity to meet challenges and recover
• Sun, light, and circadian rhythm — appropriate light exposure across the day
• Hydration and metabolic health — sustained over decades
• Environmental factors — clean air, safe water, secure shelter
• Heat and cold practices — where they fit, where culturally available

No single practice carries the whole load. The Finnish men in the sauna study were not only sauna users; they were people embedded in a culture, a family structure, a climate, a food system, a community. The sauna was part of the picture, not the whole picture.

The Junior Year View

You are 16, 17, possibly 18. You are not optimizing for longevity in 2090. You are building habits that will, if you carry them, support a long and capable life. The most powerful longevity practice at your age has nothing to do with sauna and everything to do with the ordinary:

• Move your body regularly. Build strength. Develop cardiovascular capacity.
• Eat real food. Eat enough. Get adequate protein. Eat to support a growing brain and body.
• Sleep enough and consistently. Cool, dark, quiet bedroom.
• Maintain relationships that nourish you. Avoid relationships that consistently drain you.
• Learn. Be curious about something most days.
• Reduce things you already know shorten your healthspan dramatically — nicotine, vaping, heavy alcohol use, fentanyl and other illicit substances. These are the present-tense longevity decisions for an adolescent, and they are far larger than anything heat or cold practice will ever provide.
• Wear sunscreen. Wear a seatbelt. Get sleep before late-night drives.
• Build a relationship with your body that is honest, attentive, and not punishing.

If, alongside these foundations, some form of heat practice becomes part of your life — with your family, in your culture, in your community — that is a fine addition. It is not the path. It is one of many threads.

The Camel walks slowly across a long desert. The Camel does not panic about reaching the next well. The Camel does not run. The Camel does not stop. The Camel walks, drinks when there is water, rests in the shade, walks again. Heat is not the trip; the trip is the trip. Heat is one of the things you meet along the way.

A Final Thought on Long-View Thinking

There is a particular kind of thinking that becomes available in your junior year of high school, when you are old enough to imagine your fifties and seventies but young enough that those decades feel almost mythical. Use it.

What kind of older person do you want to be? Not in terms of achievement, not in terms of wealth, but in terms of capacity — what do you want to be able to do at 50, 60, 70, 80? What relationship with your body do you want to have? What practices do you want to have carried for so long that they are simply part of who you are?

Those answers shape today's decisions in ways that nothing else does. The research on heat in adult populations is one small contribution to that long view. Coach Cold's research on cold is another. Coach Move's research on lifelong movement is the biggest. Coach Sleep, Coach Food, Coach Light, Coach Water, Coach Breath, Coach Brain — each Coach is sharing pieces of a picture that ultimately belongs to you. You are the one who decides what to do with it.

The Camel does not tell you who to become. The Camel walks alongside you while you figure it out.

Lesson Check

1. What is the difference between lifespan and healthspan, and why does the distinction matter?
2. What are two strengths of the Finnish sauna cohort study (KIHD) and two limits?
3. Why can observational research not prove that one behavior causes another outcome?
4. List five "ordinary" practices that research suggests support healthspan, regardless of whether heat practice is part of your life.
5. What is one thing about the long-view thinking in this lesson that resonates with how you would like to live?

---

End-of-Chapter Activity: A Long-View Health Map

This activity asks you to step back from the immediate and think about your relationship with your body across decades.

Step 1: Write the long view.

In 1-2 paragraphs, describe what kind of older person you want to be. Focus on capacity (what you want to be able to do), relationship (how you want to relate to your body), and practices (what you want to have been doing for decades). Do not focus on achievement, wealth, or external metrics. Be honest. There are no wrong answers.

Step 2: Map back to today.

Identify three practices that you could carry from your current age into your fifties, sixties, and beyond, that would support the version of yourself you described in Step 1. They should be ordinary, sustainable, and authentic to your life — not aspirational or extreme. Examples:

• A movement practice you can do every week regardless of where you live
• A sleep schedule that you can maintain through different life stages
• A food relationship you can carry into adulthood, family life, and travel
• A way of meeting stress that does not depend on substances or escape
• A connection to the outdoors that is part of your weekly rhythm

Step 3: Identify one challenge.

Identify one current pattern that, if left unchanged across decades, would likely undermine the version of yourself you described. This requires honest self-reflection — not punishment, but observation. What would need to shift over the coming years for this pattern not to compound?

Step 4: Where does heat (and cold) fit?

Briefly describe whether heat practice or cold practice fits into your long-view picture, and if so, in what way. There is no expectation that they must fit. Coach Hot is comfortable with the answer being "not really," or "maybe later," or "in a small way through saunas at my gym with my family," or whatever is honest for your life.

Step 5: Share with one person.

Share your long-view map with one trusted adult — a parent, an older sibling, a coach, a counselor, a mentor. Ask them what they see in it. Listen. Adjust if their observations are useful. You do not have to take their advice; you should hear it.

This activity is not graded on length or polish. It is the kind of work that benefits from being done seriously once and then revisited every few years. The Camel walks across long deserts. The map is yours.

---

Vocabulary Review

---

Chapter Quiz

Multiple Choice (1-10)

1. The gradual rise in heart rate during sustained exercise in heat at constant pace is called: A. Heat acclimatization B. Cardiovascular drift C. Hyponatremia D. Heat-induced fatigue

2. Heat as training stimulus is used by some adult athletes because: A. It replaces the need for endurance training B. It provokes similar adaptations to heat acclimatization, which can carry over to cool-condition performance C. It is the fastest way to lose weight D. It eliminates the need for cool-down

3. Core body temperature reaches its lowest point at approximately: A. Late afternoon B. Midnight C. 4-5 am D. Early morning wake-up

4. The recommended bedroom temperature range for good sleep in most adults is approximately: A. 22-25°C (72-77°F) B. 16-19°C (60-67°F) C. 10-13°C (50-55°F) D. There is no temperature-related recommendation

5. A warm bath 90 minutes before bed may help sleep because: A. It blocks dreaming B. The post-warmth core temperature drop supports sleep onset C. It increases adrenaline D. It tightens blood vessels in the skin

6. The thin layer of cells lining the inside of every blood vessel is called the: A. Myocardium B. Endothelium C. Pericardium D. Adventitia

7. Heat shock proteins are produced in response to heat stress and other stressors and primarily function to: A. Replace damaged DNA B. Refold damaged proteins, mark severely damaged proteins for removal, and protect cells from further stress C. Increase body temperature D. Bind to cell membranes only

8. Hormesis refers to: A. Hormones produced during heat exposure B. The principle that small, repeated stress produces adaptations that make the system more resilient C. A disease caused by extreme heat D. A type of inflammation marker

9. The KIHD Finnish sauna cohort study is best described as: A. A randomized controlled trial of sauna use B. A prospective observational study that followed adult men over many years and observed associations between sauna frequency and various outcomes C. A laboratory animal study D. A meta-analysis of multiple short-term experiments

10. The most important distinction Coach Hot draws when discussing the longevity research is: A. That sauna replaces exercise B. That association is not the same as causation, and the data cannot be turned into a teen prescription C. That the research applies only to Finnish people D. That the research has been overturned by newer studies

Short Answer / Application (11-15)

1. Explain why exercise in hot conditions imposes additional cardiovascular strain compared with the same exercise in cool conditions. What practical adjustments do athletic coaches commonly recommend?

2. Describe how heat exposure interacts with sleep. When during the day is heat practice generally most sleep-neutral, and why?

3. Describe how repeated heat exposure may improve endothelial function and reduce arterial stiffness in adult populations. What mechanisms have researchers proposed?

4. The Finnish sauna cohort study observed striking associations between frequent sauna use and lower rates of fatal cardiovascular events. Describe at least two strengths and two limits of this evidence.

5. Coach Hot writes that "the long view is being built now, in your habits, in your patterns, in the relationship with your body you are forming this year." In your own words, explain what this means for your daily choices and how the chapter has shaped (or not shaped) your thinking.

---

Teacher's Guide

Pacing Recommendations

This chapter is built for 8-10 class periods at 45-50 minutes each. Suggested pacing:

• Period 1: Chapter introduction + Lesson 3.1 reading
• Period 2: Lesson 3.1 discussion + cardiovascular drift demonstration (compare heart rate during a short indoor warm-up activity vs. a warmer outdoor walk if conditions allow)
• Period 3: Lesson 3.2 reading + circadian temperature curve discussion
• Period 4: Lesson 3.2 lesson check + practical sleep hygiene applications
• Period 5: Lesson 3.3 reading + endothelial function and hormesis concepts
• Period 6: Lesson 3.3 lesson check + integration with Coach Cold's hormesis lesson
• Period 7: Lesson 3.4 reading + observational vs. randomized research review
• Period 8: Lesson 3.4 lesson check + introduce End-of-Chapter Activity
• Period 9: Vocabulary review + practice quiz
• Period 10: Chapter quiz + activity discussion

Lesson Check Answers

Lesson 3.1

1. Heart rate drifts upward because skin blood flow rises to handle accumulating heat, plasma volume drifts as sweat continues, and the cardiovascular system has to do more total work over time.
2. Roughly 5-10 percent at temperatures above 25°C with significant humidity; further at higher temperatures.
3. Deliberate heat exposure to provoke heat acclimatization adaptations (plasma volume expansion, improved cardiovascular function) that can carry over to performance in cool conditions. Used by adult endurance athletes under coached programs.
4. Examples: pre-cooling, mid-cooling, hydration, pacing, acclimatization, clothing and equipment choices, scheduling around peak heat. Any three.
5. Examples: pace inexplicably slowing despite effort, confusion or dizziness, nausea, skin that stops sweating or becomes very flushed/pale, inability to follow normal conversation, racing heart with no performance change. Any three.

Lesson 3.2

1. Core temperature follows a circadian pattern: rises through morning, peaks late afternoon/early evening, falls through evening into the night, reaches lowest around 4-5 am, then rises toward waking. Sleep occurs in the cooling portion of the cycle.
2. Approximately 16-19°C (60-67°F).
3. Warm bath 90 minutes before bed allows the body to cool over the following hour-plus, and the post-warmth cooling supports sleep onset. Hot shower immediately before bed leaves the body warm at the moment of trying to sleep, which delays sleep onset.
4. Examples: close blinds during day, open windows at cool evening, use a fan, lighter bedding for the season, avoid heavy meals or vigorous exercise in last 1-2 hours before sleep.
5. Morning or early afternoon. The body has many hours to cool back down before sleep onset, so it does not interfere with the circadian temperature drop.

Lesson 3.3

1. The thin layer of cells lining the inside of every blood vessel. Crucial for blood vessel function: produces nitric oxide and other signals causing vessels to dilate appropriately, regulates inflammation, influences clotting.
2. Tends to increase with age, chronic high blood pressure, sedentary behavior, and certain disease states. Decreases with regular aerobic exercise. Has been observed to decrease with periods of repeated heat exposure in adults.
3. Heat shock proteins are produced inside cells in response to heat stress. They refold damaged proteins, mark severely damaged proteins for removal, protect cells from further stress, and modulate inflammation. Hormesis is the broader principle that a small, repeated stress produces adaptations that make the system stronger.
4. Heat does not strengthen muscles, build bones, develop coordination, or train neuromuscular skill. Exercise provides those benefits; heat provides a different set. Both have unique cardiovascular features but are not interchangeable.
5. Move your body. Get adequate sleep. Eat real food, including adequate protein. Manage stress. Maintain relationships. Avoid serious health harms (nicotine, heavy alcohol, illicit substances). Be curious and engaged.

Lesson 3.4

1. Lifespan is total years lived; healthspan is years lived in good health. A long life that ends in many years of significant illness or disability differs meaningfully from a long life that ends in years of capacity. Healthspan is often the more useful target.
2. Strengths: large sample, long follow-up, prospective design, dose-response pattern, embedded cultural practice reducing self-selection. Limits: cohort is middle-aged Finnish men, observational design cannot prove causation, possible residual confounding, generalizability questions, reverse causation possibility, no randomized trial at this scale.
3. The people who choose a behavior may differ from those who do not in many ways the researchers cannot fully measure. Statistical adjustment can reduce but not eliminate this concern. Only randomization eliminates it cleanly.
4. Examples: regular movement, adequate sleep, real food and adequate protein, social connection, cognitive engagement, stress regulation, hydration. Any five.
5. Open response. Strong answers describe a specific value or practice that resonated, articulated personally, with attention to the difference between abstract longevity goals and daily habits.

Quiz Answer Key

1. B — Cardiovascular drift
2. B — Provokes similar adaptations to heat acclimatization, which can carry over to cool-condition performance
3. C — 4-5 am
4. B — 16-19°C (60-67°F)
5. B — The post-warmth core temperature drop supports sleep onset
6. B — Endothelium
7. B — Refold damaged proteins, mark severely damaged proteins for removal, and protect cells from further stress
8. B — The principle that small, repeated stress produces adaptations that make the system more resilient
9. B — A prospective observational study that observed associations between sauna frequency and various outcomes
10. B — Association is not the same as causation, and the data cannot be turned into a teen prescription

Short Answer rubrics:

1. Strong responses identify: heat adds to the heat already generated by exercise; skin blood flow rises substantially, redirecting blood from working muscles; cardiovascular drift causes heart rate to climb. Adjustments: pacing 5-10 percent slower in significant heat, increased hydration with electrolytes, scheduled cooling and rest, modified clothing/equipment, attention to warning signs.

2. Strong responses describe the circadian temperature drop, the dependence of sleep on environmental cooling, the principle that morning/early-afternoon heat exposure does not interfere with sleep, late-afternoon heat with 90+ minutes recovery time can support sleep, and heat in the last hour before bed tends to delay sleep onset.

3. Strong responses identify: repeated shear stress on the endothelium during skin blood flow expansion; production of heat shock proteins; possible reduction in chronic low-grade inflammation; hormetic adaptation in vascular tissue. They note these effects have been observed in adult populations.

4. Strengths: scale, follow-up duration, prospective design, dose-response, cultural embedding. Limits: observational cannot prove causation; cohort specificity; possible residual confounding; reverse causation; cultural specificity.

5. Open response. Strong answers connect the long-view framing to specific habits or choices in the student's life, demonstrate honest self-reflection, and avoid either dismissiveness ("this is for old people") or overcommitment ("I'm going to sauna every day").

Discussion Prompts

1. Why might "longevity optimization" become a problematic frame even when individual recommendations are well-supported? When does long-view thinking become healthy, and when does it become anxious?
2. Many of the adaptations described in this chapter — heat shock proteins, endothelial improvement, hormesis — are also produced by exercise. What does this suggest about the relationship between heat practice and exercise?
3. Healthspan is shaped by many practices, not by any single one. If you had to rank what shapes a long, capable life in your own family or culture, what would you put first?
4. The Finnish sauna cohort findings are striking. What might explain why they have been so widely discussed in popular media — and what does the popular framing sometimes miss?
5. Heat practice for minors is descriptive, not prescriptive, in this curriculum. What other parts of health education are descriptive? Why is the descriptive frame sometimes more appropriate than the prescriptive one?
6. Coach Hot writes that "the long view is being built now." How does this framing change (or not change) how you think about decisions in your current life?
7. What is the difference between an observation in research and a recommendation you should follow? Where in your daily life do you encounter the same difference between observation and prescription?
8. Why might culturally embedded practices (like sauna in Finland) have features that "extracted" wellness practices (like sauna at a Western gym) do not? What is the difference between a practice and a tradition?

Common Student Questions

"If sauna lowers cardiovascular event rates by 50 percent, shouldn't everyone do it?" The Finnish observational data is striking, but observational does not equal randomized. The size of the effect in the cohort is not the size of the effect you would necessarily get by adding sauna to your life. The biology supports some benefit; the magnitude is uncertain.

"Could heat shock proteins help with athletic performance?" Some research has investigated this question. The current picture is that heat exposure as part of preseason or training can produce real but modest performance benefits in adult athletes. Junior-year-of-high-school athletes get most of their performance gains from training itself.

"Is the sauna-and-longevity research at all controversial?" Among researchers, the data is taken seriously but interpreted cautiously. Some researchers emphasize the strength of the findings; others emphasize the limits of observational research. This is a normal feature of how research conversations work. The honest interpretation is somewhere in the middle.

"What temperature should my room be at night?" The general recommendation is 16-19°C, but individual preferences vary. Experiment with cooler bedrooms over a few weeks and see what your sleep tells you.

"Why does my sleep get worse in summer?" Likely because your bedroom is warmer than it is in cooler seasons, and your body's natural cooling for sleep is being fought by the environment. Several practical strategies in the chapter can help.

"What about cold plunges instead of sauna?" Both train the autonomic nervous system, with overlapping but not identical adaptations. Coach Cold discusses this in their grade 11 chapter. Many traditional practices pair them. The "best" practice is the one that fits your life and that you actually do.

"How would I know if a research study is observational or randomized?" The methodology section of the paper will say. Observational studies follow people who choose their own behaviors. Randomized studies assign people to behaviors. Long-term outcomes are usually observational because randomizing long-term behavior is impractical.

"Do I need to do anything about this in my own life right now?" No. The chapter is framed as information and long-view thinking. Specific practices are decisions for your family and you to make over time.

Parent Communication Template

Subject: Coach Hot Chapter 3 — Heat as System

This chapter integrates heat physiology with the rest of the body's systems — exercise, sleep, cardiovascular adaptation across time, and longevity research in adult populations. The chapter is descriptive and educational; it does not prescribe heat protocols for minors.

Topics include why athletes pace differently in heat, why bedroom temperature affects sleep quality, what research has observed about repeated heat exposure and vascular function in adults, and the Finnish cohort research on sauna use and long-term health outcomes.

A central message throughout is the difference between observation and recommendation. The chapter helps students develop the ability to read research findings carefully, distinguish association from causation, and apply long-view thinking to their own habits without falling into anxious "longevity optimization."

The end-of-chapter activity asks students to articulate the kind of older person they want to be — in terms of capacity and relationship with their body, not achievement — and to identify ordinary habits they could carry from their current age into their fifties and beyond. This is a worthwhile family conversation.

---

Illustration Briefs

Lesson 3.1 — Cardiovascular Drift

• Placement: After "twenty minutes in, at 165 beats per minute at the same pace."
• Scene: Line graph teaching diagram. Two curves on time-vs-heart-rate axes.
• Coach involvement: Coach Hot beside the chart, calmly gesturing at the upward-drifting line.
• Mood: Educational, informative, calm authority.
• Key elements: "Cool day" curve plateaus; "hot day" curve drifts upward. Cardiovascular drift labeled. Cyan and coral lines.
• Aspect ratio: 16:9 web / 4:3 print

Lesson 3.2 — The Circadian Temperature Curve

• Placement: After "by about 0.5-1°C from its daytime peak..."
• Scene: 24-hour temperature curve. Time on x-axis, core temperature on y-axis. Smooth curve dipping at 4-5 am.
• Coach involvement: Coach Hot drowsy on a cushion in the corner, eyes half-closed.
• Mood: Quiet, evening-cool, educational.
• Key elements: Sleep window highlighted in cooler tones. Temperature low-point labeled. Warm-to-cool gradient across the day.
• Aspect ratio: 16:9 web / 4:3 print

Lesson 3.3 — The Endothelium and Repeated Stress

• Placement: After "the vessels respond by becoming better at the job."
• Scene: Cross-section of a blood vessel, with endothelial cell layer highlighted. Two side-by-side views showing the endothelium in repeat-stress state vs. baseline.
• Coach involvement: Coach Hot off to the side, watching the diagram quietly.
• Mood: Anatomical-educational, with a hint of reverence.
• Key elements: Endothelial cells clearly drawn. Tiny arrows showing shear stress from blood flow. NO labels using brand cyan and coral. Vessel wall structure visible.
• Aspect ratio: 16:9 web / 4:3 print

Lesson 3.4 — The Long View

• Placement: After "The Camel does not tell you who to become."
• Scene: Symbolic teaching scene. Coach Hot walking slowly through a stylized landscape that progresses left-to-right from young desert at sunrise through to older desert at sunset — representing a long lifespan.
• Coach involvement: Camel walking, unhurried, present at every stage.
• Mood: Reflective, calm, long-view, gentle.
• Key elements: Camel's pace consistent. Landscape changes around the figure. Warm tones throughout. Subtle visual cue (perhaps slight shift in Camel's posture between sunrise and sunset).
• Aspect ratio: 16:9 web / 4:3 print

---

Citations

1. Sawka, M. N., Leon, L. R., Montain, S. J., & Sonna, L. A. (2011). Integrated physiological mechanisms of exercise performance, adaptation, and maladaptation to heat stress. Comprehensive Physiology, 1(4), 1883-1928. https://doi.org/10.1002/cphy.c100082

2. Périard, J. D., Travers, G. J. S., Racinais, S., & Sawka, M. N. (2016). Cardiovascular adaptations supporting human exercise-heat acclimation. Autonomic Neuroscience, 196, 52-62. https://doi.org/10.1016/j.autneu.2016.02.002

3. Coyle, E. F., & Gonzalez-Alonso, J. (2001). Cardiovascular drift during prolonged exercise: New perspectives. Exercise and Sport Sciences Reviews, 29(2), 88-92. https://doi.org/10.1097/00003677-200104000-00009

4. Tucker, R., Marle, T., Lambert, E. V., & Noakes, T. D. (2006). The rate of heat storage mediates an anticipatory reduction in exercise intensity during cycling at a fixed rating of perceived exertion. Journal of Physiology, 574(3), 905-915. https://doi.org/10.1113/jphysiol.2005.101733

5. Lorenzo, S., Halliwill, J. R., Sawka, M. N., & Minson, C. T. (2010). Heat acclimation improves exercise performance. Journal of Applied Physiology, 109(4), 1140-1147. https://doi.org/10.1152/japplphysiol.00495.2010

6. Scoon, G. S. M., Hopkins, W. G., Mayhew, S., & Cotter, J. D. (2007). Effect of post-exercise sauna bathing on the endurance performance of competitive male runners. Journal of Science and Medicine in Sport, 10(4), 259-262. https://doi.org/10.1016/j.jsams.2006.06.009

7. Bongers, C. C., Hopman, M. T., & Eijsvogels, T. M. (2017). Cooling interventions for athletes: An overview of effectiveness, physiological mechanisms, and practical considerations. Temperature, 4(1), 60-78. https://doi.org/10.1080/23328940.2016.1277003

8. Casa, D. J., DeMartini, J. K., Bergeron, M. F., Csillan, D., Eichner, E. R., Lopez, R. M., Ferrara, M. S., Miller, K. C., O'Connor, F., Sawka, M. N., & Yeargin, S. W. (2015). National Athletic Trainers' Association position statement: Exertional heat illnesses. Journal of Athletic Training, 50(9), 986-1000. https://doi.org/10.4085/1062-6050-50.9.07

9. Kräuchi, K. (2007). The thermophysiological cascade leading to sleep initiation in relation to phase of entrainment. Sleep Medicine Reviews, 11(6), 439-451. https://doi.org/10.1016/j.smrv.2007.07.001

10. Okamoto-Mizuno, K., & Mizuno, K. (2012). Effects of thermal environment on sleep and circadian rhythm. Journal of Physiological Anthropology, 31(1), 14. https://doi.org/10.1186/1880-6805-31-14

11. Harding, E. C., Franks, N. P., & Wisden, W. (2019). The temperature dependence of sleep. Frontiers in Neuroscience, 13, 336. https://doi.org/10.3389/fnins.2019.00336

12. Haghayegh, S., Khoshnevis, S., Smolensky, M. H., Diller, K. R., & Castriotta, R. J. (2019). Before-bedtime passive body heating by warm shower or bath to improve sleep: A systematic review and meta-analysis. Sleep Medicine Reviews, 46, 124-135. https://doi.org/10.1016/j.smrv.2019.04.008

13. 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. https://doi.org/10.1016/j.mayocp.2018.04.008

14. Vita, J. A., & Keaney, J. F. (2002). Endothelial function: A barometer for cardiovascular risk? Circulation, 106(6), 640-642. https://doi.org/10.1161/01.CIR.0000028581.07992.56

15. Brunt, V. E., Howard, M. J., Francisco, M. A., Ely, B. R., & Minson, C. T. (2016). Passive heat therapy improves endothelial function, arterial stiffness and blood pressure in sedentary humans. Journal of Physiology, 594(18), 5329-5342. https://doi.org/10.1113/JP272453

16. Lee, E., Laukkanen, T., Kunutsor, S. K., Khan, H., Willeit, P., Zaccardi, F., & Laukkanen, J. A. (2018). Sauna exposure leads to improved arterial compliance: Findings from a non-randomised experimental study. European Journal of Preventive Cardiology, 25(2), 130-138. https://doi.org/10.1177/2047487317737629

17. Kampinga, H. H., & Craig, E. A. (2010). The HSP70 chaperone machinery: J proteins as drivers of functional specificity. Nature Reviews Molecular Cell Biology, 11(8), 579-592. https://doi.org/10.1038/nrm2941

18. Lavie, C. J., Ozemek, C., Carbone, S., Katzmarzyk, P. T., & Blair, S. N. (2019). Sedentary behavior, exercise, and cardiovascular health. Circulation Research, 124(5), 799-815. https://doi.org/10.1161/CIRCRESAHA.118.312669

19. Laukkanen, T., Khan, H., Zaccardi, F., & Laukkanen, J. A. (2015). Association between sauna bathing and fatal cardiovascular and all-cause mortality events. JAMA Internal Medicine, 175(4), 542-548. https://doi.org/10.1001/jamainternmed.2014.8187

20. Laukkanen, T., Kunutsor, S., Kauhanen, J., & Laukkanen, J. A. (2017). Sauna bathing is inversely associated with dementia and Alzheimer's disease in middle-aged Finnish men. Age and Ageing, 46(2), 245-249. https://doi.org/10.1093/ageing/afw212

21. Kaeberlein, M. (2018). How healthy is the healthspan concept? GeroScience, 40(4), 361-364. https://doi.org/10.1007/s11357-018-0036-9

22. Kunutsor, S. K., Khan, H., Zaccardi, F., Laukkanen, T., Willeit, P., & Laukkanen, J. A. (2018). Sauna bathing reduces the risk of stroke in Finnish men and women: A prospective cohort study. Neurology, 90(22), e1937-e1944. https://doi.org/10.1212/WNL.0000000000005606

23. Hannuksela, M. L., & Ellahham, S. (2001). Benefits and risks of sauna bathing. American Journal of Medicine, 110(2), 118-126. https://doi.org/10.1016/S0002-9343(00)00671-9