The slump many people feel around 40 is a real collision, not a mood. Michelle Spear, professor of anatomy at the University of Bristol, calls this the most exhausting decade of life: biology is getting more expensive just as work and family demand the most. The useful part of her account is that the dip is not permanent. By 60 a lot of people get a second wind.
Recovery starts to cost more
Spear describes the 40s as a mismatch between biology and what the day asks. In your 20s, mitochondria, the cell's power stations, covered late nights and skipped training. Around 40 they make less energy and leave more inflammatory waste. Every repair job gets pricier.
Four drains that land at once
- Cortisol stops falling at night. Sleep stays light and fails to restore you.
- This is the decade of the senior job, children, and aging parents. The mental load tires the brain the way heavy work tires muscle.
- From about 35 to 39, muscle mass starts to shrink, so ordinary movement takes more effort than it did ten years earlier.
- In perimenopause, swings in estrogen and progesterone hit deep sleep and temperature control.
In your 20s the body is biologically forgiving: muscle recovers faster and inflammation is brief. At 40 that changes, Spear says.
The second wind around 60
Researchers who follow people past this decade say the load often eases. Stress tends to drop, the schedule loosens, and sleep settles. Studies also find that regular strength work even in your 70s can rebuild metabolic health and bring energy back toward a younger level.
The practical advice is not to chase a 20-year-old's output. Sleep, enough protein, and lifting weights are what keep muscle, and energy, through the hardest decade.
