I'm not as tired as I was yesterday, but I remember some other times of brain-driven exhaustion. I described yesterday how my brain can make me work young, like, oh, a fifty-year old. Here's another example of how I've driven myself like a young'un, recovering more and more slowly as I age.
I do a few really exhausting things once or twice a year, and I've been doing them for many years: I lead some of the Orthodox Jewish prayer services for Rosh Hashana and Yom Kippur. The goal of a good prayer service is entirely different from an opera production, but the demands are similar: you're on your feet and singing with dramatic intent for long periods of time.
Leading these services calls for lots of energy. The closing service of Yom Kippur, which starts after 24 hours of neither eating nor drinking, is particularly demanding. I started leading these services many years ago, so I know how to husband my strength. But I don't really have that strength anymore, it's just a feat of mind over body. I remember, about twenty years ago, an undergraduate leading a Yom Kippur service I had led the previous year. I could see him tire as the service went on. Afterward, he asked me, “Where do you get the energy?” I could tell him then; I can't tell him now, but that energy comes from somewhere when I need it.
The words are the same each year; the melodies are similar. As I harness myself to these familiar patterns, I feel the energy flow. It's incredible to know that I can call on all that strength; and fascinating to know that I'll spend days afterwards, paying my body back. All of you are going to have similar experiences. All it takes is a vigorous activity you don't want to leave behind you.