Doing a thing efficiently is yet another factor adding to our credit of time and energy. If any work, may be accounts, typing or any work for that matter, is done efficiently, our time and energy is doing the same thing over and again, are definitely saved and we get so much of time and energy for other useful work. Yogasanas is an energy-storing process. If somebody gets exhausted and drowsy after Yogasanas it indicates that something is wrong somewhere. One must necessarily practice Pranayama and meditation for the health of the mind.
The intellect does not lose energy. By rightly using it, it gets sharpened and more efficient. For this, we must have Swadhyaya.
Swadhyayodhyetavyah Swadhyaya pravachanabhyam na pramaditavyam.
....Taittiriya Upanishad Shishvalli
The brain is never overworked or taxed. It is a false notion. In Swadhyaya, one must read thought-provoking literature like the Gita, the Upanishads and other sacred texts. Knowledge when given never decreases, but on the contrary, it increases.
Vyaye krite vardhate eva nityam - Subhashita
[When spent it (knowledge) always increases]. It is not lost when imparted, but is multiplied like the flame of the lamp which brings in more illumination by lighting other lamps without losing its own luster.