I have noticed, many times, that after being engaged in deep thought, especially deeply creative thought, that I would sometimes feel very fatigued. It occurs to me, that from an energy standpoint, our bodies are only able to generate a certain number of calories of energy per day. So therefore, we can only think so much in a day; we can only create so much in a day. And beyond that, all thoughts are not equal. A deeply complex thought that calls all one's experience and knowledge into play, must certainly burn more calories than a simple thought of recognition such as: "This is cheese," or, "This is water."
But when one realizes a revelation, such as suddenly understanding the principle of the speed of light, there is not only expended the caloric energy of thought to reach that realization, but there is often a corresponding surge of energy due to elation. Thus, life-changing thoughts and experiences must truly burn the most calories and therefore be the most taxing.
Perhaps that is one of the reasons that many great thinkers have been very lean. They not only aren't concerned with pleasing themselves through the sense of taste, but they also are burning more calories by thinking vigorously.
Voltaire immediately springs to mind, and the unbelievable number of manuscripts which he produced. "Voltaire left behind him over fourteen thousand known letters and over two thousand books and pamphlets." I can't even begin to imagine how anyone could write so profusely. (see link below)