We know sleep, exercise and diet are vital for health. But why?

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Sleep. Exercise. Eat well. It’s the oldest recommendation in medicine, repeated so often it’s become a cliche. We know the advice, even if we don’t always follow it. But have you ever stopped to wonder why these three seemingly different habits have such remarkably similar effects on our brains and bodies?

For generations we’ve treated them as separate prescriptions – sleep restores us, exercise strengthens us and food fuels us. Yet neuroscience is beginning to suggest something rather more elegant: they’re all essentially doing the same job.

What has changed is not the recommendations themselves, but our understanding of the biology beneath them. The more scientists study the brain, the clearer it becomes that cognition is astonishingly energy intensive. Although the brain makes up about 2% of body weight, it uses roughly a fifth of the body’s energy at rest. Every memory we retrieve, every decision we make and every creative insight carries a metabolic cost. Thought itself is an energy-hungry act.

That raises an obvious question: where does all that energy come from? The answer lies deep in evolutionary history. More than 2bn years ago, a primitive cell engulfed a bacterium. Instead of being digested, the bacterium remained, forming a long-term partnership with its host.

Over time, this arrangement became one of biology’s most important alliances. The bacterium became remarkably efficient at capturing the chemical energy stored in food and transforming it into ATP, the molecule that powers almost every biological process. The host offered protection, and together they made complex life possible.

That partnership is still running inside us today. Mitochondria – the descendants of that ancient bacterium – exist in almost every cell in the body. The exception is the mature red blood cell, which removes them to make space for haemoglobin and improve oxygen transport. Even here, biology reveals its logic: everything is shaped by trade-offs in energy efficiency.

For decades, mitochondria were introduced in biology textbooks as the cell’s “powerhouses” – important, dependable but rather dull. The stars of the show were undeniably neurons. Today, that picture is changing.

We’re discovering that neurons produce thought, ideas and emotions only because mitochondria continuously transform fuel into the energy they need. Intelligence depends not simply on the brain’s wiring, but on the uninterrupted flow of energy through that wiring. This is a subtle shift in perspective, but an important one.

Emerging evidence suggests that healthy mitochondrial function helps give rise to intelligence, including driving arithmetic and processing speed, working memory, healthier brain ageing and greater resilience to stress. Mitochondria are no longer viewed as passive batteries but as active participants in the delicate balancing act that allows the brain to adapt, learn and flourish throughout life.

Suddenly, the oldest health advice begins to make much more sense. Exercise doesn’t simply boost muscle mass, it bulks up the hippocampus – a key region of the brain involved in learning and memory. It also encourages mitochondria to become more numerous and more efficient. We often imagine we’re training our bodies; we’re also strengthening the ancient energy system that allows our brains to work.

Sleep performs a different kind of maintenance. While we’re unconscious, the brain is astonishingly busy. Memories are consolidated and worn-out cellular components – including mitochondria – are repaired or replaced. So rather than thinking of sleep as simply rest, we can conceive of it, at least partly, as a mitochondrial maintenance shift, aimed at preparing the brain for another day of active learning, adapting and imagining.

Then there’s food. Mitochondria rely entirely on what we eat to perform their remarkable chemistry. The quality of that fuel matters. Diets rich in minimally processed foods provide the ingredients for efficient energy transformation, while diets that continually overwhelm the system appear to make that process less efficient over time.

Seen through this lens, sleep, exercise and diet stop looking like three separate pieces of lifestyle advice and become different ways of caring for the same biological inheritance. What fascinates me most is that this new understanding reaches far beyond physical health. Scientists are uncovering intriguing links between mitochondrial function and resilience, mood, personality traits, and even our social lives.

For centuries we’ve tended to separate mind from body, as though thoughts somehow floated free of the flesh that produced them. The reality is that thoughts emerge from living tissue that must constantly capture, transform and use energy simply to exist.

The mitochondrial psychobiologist Martin Picard has argued that we’ve become accustomed to thinking of ourselves as molecular machines, when it may be more useful to think of ourselves as energetic processes. The brain is an organ whose every thought, memory and feeling depends on the continuous transformation of energy within billions of cells.

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Seen this way, sleep is no longer simply rest, and exercise is no longer simply fitness. Food is no longer simply fuel. They’re different ways of sustaining the ancient biological partnership that allows our bodies – and our minds – to flourish.

It’s remarkable to think that every idea you’ve ever had, every problem you’ve solved, every person you’ve loved and every hope you’ve held has depended upon an alliance forged more than 2bn years ago. The tiny descendants of that ancient bacterium have been quietly working inside us ever since, making possible not just movement or metabolism but thought itself.

Perhaps that’s why the oldest health advice has survived every new scientific fashion. Long before we knew about ATP, oxidative phosphorylation or mitochondria, we recognised that people who slept well, moved regularly and ate nourishing food tended to flourish. We’re finally beginning to understand why.

Hannah Critchlow is a neuroscientist and author of The 21st Century Brain (Torva).