Introduction

We often envision our planet as a vibrant "blue planet," rich and abundant in its resources. However, in reality, humans are confined to utilising only about 15% of the Earth's surface. The remainder consists of water, mountains, ice, or deserts—inhospitable areas for human habitation.

Mike Tipton, a university professor at the University of Portsmouth in the UK, explains that humans, originally tropical animals, constantly face environmental challenges. Having settled in regions far beyond our warm ancestral climate, we are in a continuous state of adaptation, navigating the unique demands of diverse environments.

One of the fundamental challenges for all living beings is maintaining an adequate internal temperature. Through evolution, organisms have developed sophisticated systems to maintain this balance and survive even in the most extreme environments. Humans are no exception. Evolution has equipped us with a complex interplay of mechanisms—some inherited from earlier organisms and others unique to humans—that maintain our core temperature within a narrow range, regardless of external or internal conditions.

We relied heavily on these natural thermoregulatory systems for much of human history. However, our species' larger prefrontal cortex enabled us to invent tools, control fire, create clothing, and eventually develop air conditioning. These innovations brought comfort, made survival more manageable, and introduced new complications. Suddenly, we no longer need as much energy to obtain food, traverse distances, or maintain our inner temperature. With the advent of efficient clothing and climate-controlled environments, our thermoregulatory systems have become less necessary and, consequently, less responsive.

While these advancements have undoubtedly enhanced our quality of life, rendering it more comfortable, convenient, and arguably more enjoyable, they have come at a cost. Evolutionary processes tend to phase out or diminish functions that are no longer critical for survival. For instance, we have lost our tails and the ability to move our ears towards sounds, changes that have little impact on our health or survival. However, the diminished capacity to adapt to thermal stress carries far more serious consequences.

Thermal regulation is a core and essential homeostatic function in homeothermic animals, including humans. [1] It requires the body to maintain a relatively stable internal temperature. As we will explore later, this function is intricately linked to many other metabolic processes. When it weakens, other metabolic processes can also decline, compromising overall health.

In essence, the comfort we so eagerly pursue can weaken our bodies, foster disease, and ultimately shorten our lives. Recognising this trade-off is a critical takeaway—and perhaps the first meaningful message of this book. Maintaining and nurturing our thermoregulatory capacity is crucial for maintaining good health and living longer, more robust lives.

Of course, abandoning comfort or progress is neither realistic nor necessary. Humans have always sought to reduce strain and conserve energy, a natural inclination rooted in our evolutionary past. Advancements like artificial intelligence and accelerated automation are poised to diminish further the challenges our bodies face in meeting their fundamental physiological needs.

We have long understood the health risks of sedentary lifestyles and poor diets, which are widely acknowledged as leading contributors to chronic diseases. [2] Far less attention has been given to the absence of other challenges that shaped our evolutionary development. For instance, the lack of exposure to natural temperature fluctuations—and its potential consequences—remains an underexplored topic.

Few people consider the implications of living in temperature-controlled environments, and the scientific literature on this issue is surprisingly sparse. This might create the impression that it is overlooked, but that is not the case. Substantial evidence highlights the benefits of short-term exposure to heat or cold stress, as well as the adaptive mechanisms triggered by such experiences. From this, we can infer a critical insight: if exposure to thermal stress is beneficial, then its absence is likely to have adverse effects.

These adverse effects can be severe, even fatal, particularly among vulnerable populations. For example, impaired thermoregulation, especially in the elderly, contributes to thousands of premature deaths each year. [3] This issue is further compounded by environmental changes and the increasing frequency of extreme weather events, which are expected to exacerbate these challenges. [4]

Understanding the importance of reintroducing controlled thermal stress into our lives while still embracing the benefits of modern advancements could be a crucial step toward addressing this often-overlooked aspect of health and well-being.

The sauna enhances the body's thermoregulatory mechanisms, especially when paired with a cold bath. This combination strengthens the body's ability to adapt to temperature fluctuations and improves a wide range of physiological processes closely tied to thermoregulation. Regular sauna and cold therapy can stimulate these systems, contributing significantly to overall health and resilience.