Caffeine and conscious effort to resist it can last you a while. If you practice maybe you last longer/better, similar to a cognitive form of running.
Even the best runners can't go forever, though.
This sounds like one of those "ensure this situation does not come to pass" type of things.
Said as if such were consistently possible...
Actually, I'm probably a pretty good test case. Active in high school, near top of class + 9 varsity letters, community service etc. When I got to college, everything became gradually harder and I had less energy. I started keeping weird sleep hours too. People who I was close to turned on me because I wasn't as successful, and relatives started to think I was lazy/not trying...perhaps a fair assumption since my actual actions/performance declined. Emotionally, I was pretty resistant for a few years but eventually that caught up to me and it shaped my world view. Nothing is really unconditional when it comes to relationships, of any kind. I should know, I lost at least one of each standard kind across this period and only got some of them back later.
Later I had blood work and it turned out my thyroid was completely shot, one of the highest TSH values they'd ever seen. Magically I'm really strong academically in graduate school again (wonder why...), but still carry some lingering stigma from how I was in undergrad from people who knew me before/during, as if I just made a mistake then or something rather than fighting through a serious medical deficiency.
Does this influence your personality? No question. Both how others treat you and your own energy levels. I lost nearly all sense of direction in the space of two years, after that it was like starting over (but I was still not tested for another two years). I do know you can resist fatigue for periods. I think it's a little strange to have someone reach decompensation routinely just from being tired, but with non-descript "health issues"? Yeah, I can see it. Whether it's realistically controlled depends on the situation.
One can always do a little better, but the specifics influence whether it's more like asking someone to exercise sometimes or asking someone to chain marathons back to back rather than just running one.