I do experience a malaise after exercise or stress, but I'm not sure if it would be classified as post-exertional malaise present in CFS. I'm interested in how others experience it so I can better understand what's going on. A description of how I experience it follows: I'm reluctant to initiate physical activity, but once I get going, it does feel good. Somehow I don't tolerate exercise well though, because afterwards or even during I will suddenly notice that I'm starting to feel worse. I'm unable to recognize that I'm going over my limits. I simply don't feel that anything is wrong until it's too late. It is not just physical activity, anything that requires concentration, is stressful or exhausting can cause it. The explanation that appears to explain these observations best is that my anti-oxidant reserves are low and thus any form of stress can cause harmfull oxidative stress after the anti-oxidant reserves have been used up (this is pretty much the methylation problems theory as far as I understand it). What the malaise feels like? Rapid drop in body temperature (this goes away after a nap) Sudden feeling of intense stress, unease, and depression and generally feeling miserable. Loss of productivity and ability to concentrate. Pain in the lower back. Low energy levels. How long does it last? It's proportional to the intensity of the activity/stress. Hours to a few days. A negative effect could linger for much longer after intense exertion or stress but I cannot clearly distinguish that from "normal" fluctuations of symptoms (which are presumably already reflecting stressors).