Somewhat related to desires above, this one cites a few research papers that seem to indicate that we don’t burn a lot of energy when doing intensive work with our brains. The fatigue we sense has to do with losing motivation and being surrounded by too many temptations (hello internet!).
Instead, they found this small correlation: The nurses who were least likely to feel fatigued from their work also felt the most in control of their work, and the most rewarded for it. These feelings may have boosted their motivation, which may have boosted their perception of having energy.