This is true for me as well. Whenever my sleep starts to get really bad then I have to rest a lot more for about a week for it to get back to my "normal" sleep. Even my current level of "normal" sleep is not great (waking up 6-7 times a night, never getting more than 1.5 hours of sleep at a stretch). But when my sleep gets to "horrible" (waking up 12-14 times per night, trouble breathing, feeling overheated, and finally getting out of bed because my heart beats faster lying flat than it does while sitting in the recliner) then I know that means it is time to rest a lot more--which I hate, but I have not found another option. My sleep got bad last summer but I've been pretty careful since then to rest a lot and it's back to my normal level. I'm lucky that I fall asleep very quickly. I actually don't take any sleep medications. And even after I wake up in the night I can even fall back asleep without any problem (fixed a major typo) (no worrying or anxiety, for example). It's just annoying that I can't sleep more hours in a row. It's a paradox to most folks that resting during the day more makes my sleep better. My doctor explained that, in my case at least, it is autonomic problems that are causing a lot of my sleep problems. When I do things during the day that aggravate my NMH (Neurally Mediated Hypotension) then it messes up my sleep. And this can accumulate over time somehow (not sure how). The meds that I take for NMH are helpful but they are not a substitute for resting and avoiding NMH triggers. Edited to add - I hope this was not too much of a tangent from the original question about post exertional exhaustion and how to predict it. I have never been able to calibrate it down to the level of "x minutes of activity is okay" or when the PEM kicks in. For me it is more about avoiding those things that cause problems (standing, heat) than it is about the number of minutes of activity per day. I just have to play it by ear and take it one day at a time.