Rethinking Post-Exercise Fatigue vs Post-Exertional Sickness

I've been classifying the fatigue lasting less than 48 hours as post-exercise fatigue rather than post-exertional sickness. I reasoned that a day of fatigue after a heavy activity is normal even for a healthy person. It is a happy fatigue flush with endorphin. But the fatigue on 10/3 was not a happy fatigue even though it lasted only a day; it was a sick, wallowing fatigue. That made me wonder if the duration was the correct criteria for the distinction, or if the distinction is meaningful at all.

Frequent post-exertional sickness means  that you are over-reaching. And knowing that is imperative in managing CFS. So the duration and frequency of post-exertional sickness must be measured and recorded. But the normal post-exercise fatigue should not be included in it because, well, it is normal. More importantly, post-exercise fatigue is not disabling; my mind actually works better when I'm having post-exercise fatigue. So the distinction between post-exertional sickness and post-exercise fatigue is still meaningful and useful.

The problem is, again, there are one-day fatigues that are not happy post-exercise fatigues: the one-day-up-next-day-down pattern during a post-trip struggle is not post-exercise fatigues even though they last one day. So the duration-base classification can end up with false negatives (for post-exertional sickness) and under-register the struggle. I could remedy the problem by adding the happy/sick feeling as a criteria, but that is yet another subjectivity that I don't need.

Since post-exercise fatigue usually last one day and post-exertional sickness usually multiple days, the current classification by the duration is probably still useful enough. I'll stick to it for now till I find a better way to cull out the post-exercise fatigue from the post-exertional sickness. For post-trip struggles, enough multi-day post-exertional sickness also occur, so ignoring one day sickness may not be a big problem.