If I'm absolutely exhausted at the end of the day I know that I have been truly teaching. Standing in front of a class and lecturing for five hours isn't that big of a deal. Constantly walking around the room, from group to group, student to student and answering specific questions, troubleshooting problems, and mediating student conflict is exhausting.
Truly teaching someone take personal time, attention, and effort. Anyone who says otherwise hasn't truly taught.