It's the discipline vs passion effect. Discipline can only power you so much. It taps from your willpower reserves, which is limited, and then wait for your willpower to recover.
Passion is the opposite, you have to expend willpower to not do the thing. Once you do the thing, you end up with more energy.
Passion can be destructive in many situations though, especially jobs where your work may be writing docs, meetings, demos, and not doing the thing. So highly passionate people in "the corporation" end up feeling more drained than disciplined people.
But basically what you're seeing is a comparison between the two, instead of seeing it as a normal thing like the sleep/wake cycle.
Passion is the opposite, you have to expend willpower to not do the thing. Once you do the thing, you end up with more energy.
Passion can be destructive in many situations though, especially jobs where your work may be writing docs, meetings, demos, and not doing the thing. So highly passionate people in "the corporation" end up feeling more drained than disciplined people.
But basically what you're seeing is a comparison between the two, instead of seeing it as a normal thing like the sleep/wake cycle.