Sleep, exercise, and a healthy diet are the oldest recommendations in medicine, repeated so frequently they have become cliches. While most people are familiar with this advice, few consider why these three distinct habits produce such similar benefits for both the body and the brain. For generations, these behaviors were treated as separate prescriptions—sleep for restoration, exercise for strength, and food for fuel. However, modern neuroscience suggests a more elegant reality: they are all performing the same essential task.
The shift in perspective stems from a deeper understanding of our biology. Cognition is an incredibly energy-intensive process. Although the brain accounts for only about 2% of total body weight, it consumes roughly one-fifth of the body’s resting energy. Every memory retrieved, decision made, and creative thought formed requires a significant metabolic investment. Thinking itself is an energy-hungry act.
This energy demand traces back to an evolutionary milestone that occurred more than 2 billion years ago, when a primitive cell engulfed a bacterium. Instead of being digested, the bacterium formed a long-term partnership with its host. This ancient alliance allowed the bacterium to capture chemical energy from food and transform it into ATP, the molecule that powers nearly all biological processes. This partnership remains active within us today through mitochondria, the descendants of that ancient bacterium.
Mitochondria exist in almost every cell in the human body, with the notable exception of mature red blood cells, which discard them to create space for hemoglobin and improve oxygen transport. For decades, biology textbooks dismissed mitochondria as merely the cell’s “powerhouses”—important, yet dull. Today, that view is shifting. Researchers now recognize that neurons produce thoughts, emotions, and ideas only because mitochondria continuously convert fuel into necessary energy. Intelligence relies on the uninterrupted flow of energy through the brain’s complex wiring.
Emerging evidence indicates that healthy mitochondrial function supports intelligence, including arithmetic processing, speed, working memory, resilience to stress, and healthier brain aging. Mitochondria are now viewed as active participants in the brain’s ability to adapt and flourish. This explains why traditional health advice is so effective. Exercise, for instance, does more than build muscle; it expands the hippocampus, a brain region critical for memory and learning, while encouraging mitochondria to become more numerous and efficient. We are not just training our bodies; we are strengthening the energy systems that power our minds.
Sleep serves as a different form of maintenance. During sleep, the brain is highly active, consolidating memories and repairing or replacing worn-out cellular components, including mitochondria. It acts as a maintenance shift, preparing the brain for the next day of learning and imagination. Meanwhile, diet provides the raw materials for mitochondrial chemistry. Minimally processed foods offer the ingredients required for efficient energy transformation, whereas diets that overwhelm the system can degrade that efficiency over time.
By viewing these habits through this lens, sleep, exercise, and diet are no longer separate lifestyle tips but different ways of caring for the same biological inheritance. Scientists are uncovering links between mitochondrial function and broader aspects of human life, including mood, personality traits, and social interactions. As neuroscientist and author ofThe 21st Century BrainHannah Critchlow, notes, we have historically separated the mind from the body, but thoughts emerge from living tissue that must constantly capture and transform energy to exist.
Mitochondrial psychobiologist Martin Picard has argued that it may be more accurate to view ourselves as “energetic processes” rather than mere molecular machines. Every thought and feeling depends on this continuous energy transformation within billions of cells. It is remarkable to consider that every idea, problem solved, loved one, and hope held has depended upon an alliance forged more than 2 billion years ago. These tiny descendants of an ancient bacterium have been working inside us ever since, making thought itself possible.
This explains why the oldest health advice has survived every scientific trend. Long before we understood ATP or oxidative phosphorylation, humans recognized that those who slept well, moved regularly, and ate nourishing food tended to flourish. We are finally beginning to understand why.
Further reading on these topics can be found inThe Song of the Cellby Siddhartha MukherjeeTransformer: The Deep Chemistry of Life and Deathby Nick Lane, andHow Life Works: A User’s Guide to the New Biologyby Philip Ball. The report also notes that the Song of the Cell by Siddhartha Mukherjee (Vintage, £14.99). The report also notes that transformer: The Deep Chemistry of Life and Death by Nick Lane (Profile, £14.99. The report also notes that how Life Works: A User’s Guide to the New Biology by Philip Ball (Picador, £14.99).
