Some of us remember having more energy in our 20s. We could work late, sleep badly, have a night out, recover quickly and still feel capable the next day. By our 40s, that ease has often gone. Fatigue feels harder to shake. It’s tempting to assume this is simply the ageing process – a one‑way decline.

The truth is that the 40s are often the most exhausting decade, not because we are old, but because several small biological changes converge at exactly the same time that life’s demands often peak. Crucially, and optimistically, there is no reason to assume that energy must continue to decline in the same way into our 60s.

In early adulthood, multiple systems peak together.

Muscle mass is at its highest, even without deliberate training. As a metabolically active tissue, muscle helps regulate blood sugar and reduces the effort required for everyday tasks. Research shows that skeletal muscle is metabolically active even at rest and contributes substantially to basal metabolic rate (the energy your body uses just to keep you alive when you’re at rest). When you have more muscle, everything costs less energy.

At the cellular level, mitochondria – the structures that convert food into........

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