I have described to you how this model explains the way people invest their focus, as a given quantity. The model goes a little farther in penetrating human nature as it explains the degeneracy of sanity that almost all humans undergo while ageing.
This model does not interest itself in the physical degeneracy of the brain (and body), caused by genetic timing. It first needs the reader to accept that young persons (but past puberty, or before it) are more likely to be sane than older persons. Older persons more often behave in an irrational pattern admittedly due to the fact that Life leaves no one unscarred, and that some of our toughest experiences create paranoia, neurosis and other psychological anomalies that shake a little bit our rationality. A young person can be expected to have never lived such experiences and is very likely to react in the most rational way to a given stimulus.
This model has a different approach to the issue.
It gets a little sciency for a short while.
If the laws of thermodynamics can be applied to the process of reconstruction of the fluid capital over sleeping, the entropy of this process must be negative. And the process of spending it by focussing is obviously of positive entropy (if focus can be assimilated to "work"). It is then not foolish to assume that a successful reconstruction of the capital would take it back to it's exact previous-day-level, minus an infinitesimal quantity.
One wouldn't notice a difference from one day to the next, but over the years, it might make a difference. That would explain the reduced focus capabilities of the, let's say forty years old compared to that of the, let's say twenty years old.
And, as we are into defining "focus" as a physical quantity, then it follows very reasonably that the practice of an activity demanding a lot more focus than that available would cause damage to the whole apparatus. Damage that would be repairable in a few days by a few good nights of sleep if not too drastic, or repairable only to a certain extent.
Here is taken into account that some very stressful events of peoples lives affect their behaviour for some time, possibly for ever.