The health industry sells you everything that can be packaged: supplements, apps, memberships, regimens. Notice what it sells you nothing for - sleep. And that is precisely the thing that internal medicine specialist Maria Fernandez from the Neleva clinic says ages you faster than any missed workout, and that the body registers after a single bad night.

The sentence sounds exaggerated until you look at what actually happens while you sleep. The night splits into two halves and each has its own job. In the first half, in deep sleep, growth hormone is released to repair tissue, and the glymphatic system switches on - the channel through which the brain flushes out waste. In the second half comes REM, when memory is consolidated, especially emotional memory and learned skills.

When the shortfall becomes a habit, the bill is specific

The brain struggles to clear the waste. Blood sugar control worsens. Cortisol rises. Immunity weakens, and in the background a low-grade inflammatory state quietly persists - the kind that does not hurt and cannot be seen, but does not stop either. A single insufficient night measurably alters cortisol, glucose and insulin.

The most useful thing Fernandez says is what many people do not want to hear: training does not cover the debt. Quite the opposite. "Training after poor sleep carries an added risk - injuries become more frequent, because muscle repairs itself while at rest," she says. So a gym session after three hours of sleep is not compensation, it is a second blow.

The list is free, which is probably why nobody advertises it

The same bedtime and waking hour, weekends included. Dinner two to three hours before sleep. No screens in the last hour. A cool, dark, quiet room. No alcohol or caffeine in the afternoon. Breathing or light stretching before bed.

Not one item costs a penny. Not one can be packaged in a bottle with a label. Is that a coincidence, or is it the answer to why we hear least of all about sleep?