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. 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 fewer inflammatory byproducts.
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.
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.