My guess: by measuring the minimality of the amount of energy spent restoring neuron depolarization capability by way of efficient clearance of metabolic waste (and wider network adaptivity) of thalamo-cortical circuits. The hypothesis being: 'for a neuron to struggle to repolarize = thinking feels and works bad'
How to improve it? Good hydration, good vascular system architecture, good sleep, good hemodynamics, low tension, low inflammation, good glymphatic drainage during sleep, good interspersed periods of rest between thinking, good perception of one's own fatigue so you don't burn yourself further with more activity - or even better, so you know how to make yourself feel at ease and comfortable to think, etc.
https://neurosciencenews.com/thalamus-conscious-perception-2...
