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.
Energetic 20s
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 usable energy – are more numerous and more efficient. They produce energy with less waste and less inflammatory byproduct.
Sleep, too, is deeper. Even when sleep is shortened, the brain produces more slow‑wave sleep, the phase most strongly linked to physical restoration.
Hormonal rhythms are also more stable. Cortisol, often described as the body’s stress hormone, melatonin, growth hormone and sex hormones follow predictable daily patterns, making energy more reliable across the day.
Put simply, energy in your 20s is abundant and forgiving. You can mistreat it and still get away with it.
Exhausting 40s
By midlife, none of these systems has collapsed, but small shifts start to matter.
Muscle mass begins to decline from the late 30s onwards unless you exercise to maintain it. This in itself is a top tip – do strength training. The loss of muscle is gradual, but its effects are not. Less muscle means everyday movement costs more energy, even if you don’t consciously notice it.
Mitochondria still produce energy, but less efficiently. In your 20s, poor sleep or stress could be buffered. In your 40s, inefficiency is exposed. Recovery becomes more “expensive”.
Sleep also changes. Many people still get enough hours, but sleep fragments. Less deep sleep means less repair. Fatigue feels cumulative rather than episodic.
Hormones don’t disappear in midlife – they fluctuate, particularly in women. Variability, not deficiency, disrupts temperature regulation, sleep timing and energy rhythms. The body copes better with low levels than with unpredictable ones.
Then there is the brain. Midlife is a period of maximum cognitive and emotional load: leadership, responsibility, vigilance and caring roles. The prefrontal cortex – responsible for planning, making decisions and inhibition – works harder for the same output. Mental multitasking drains energy as effectively as physical labour.
This is why the 40s feel so punishing. Biological efficiency is beginning to shift at exactly the moment when demand is highest.
Krakenimages/Shutterstock.com
Hopeful 60s
Later life is often imagined as a continuation of midlife decline; however, many people report something different.
Hormonal systems often stabilise after periods of transition. Life roles may simplify. Cognitive load can reduce. Experience replaces constant active decision‑making.
Sleep doesn’t automatically worsen with age. When stress is lower and routines are protected, sleep efficiency can improve – even if total sleep time is shorter.
Crucially, muscle and mitochondria still adapt surprisingly well into later life. Strength training in people in their 60s, 70s and beyond can restore strength, improve metabolic health and increase subjective energy within months.
This doesn’t mean later life brings boundless energy, but it often brings something else: predictability.
Good news?
Across adulthood, energy shifts in character rather than simply declining. The mistake we make is assuming that feeling tired in midlife reflects a personal failing, or that it marks the start of an unavoidable decline. Anatomically, it is neither.
Midlife fatigue is best understood as a mismatch between biology and demand: small shifts in efficiency occurring at precisely the point when cognitive, emotional and practical loads are at their highest.
The hopeful message is not that we can reclaim our 20-year-old selves. Rather, it is that energy in later life remains highly modifiable, and that the exhaustion so characteristic of the 40s is not the endpoint of the story. Fatigue at this stage is not a warning of inevitable decline; it is a signal that the rules have changed.
