The problem with PEM and current science is that it is still a syndrome. We don't understand it. I suspect its a composite symptom cluster with multiple issues, each of which has their own time scale. Some aspects of it are being unravelled though, from mitochondrial and delayed energy production issues, to more immediate muscle lactate responses. Given that this involves metabolism, nerve signals, hormonal signals and immunological signals, each with its own time frame, the way to think of PEM might be one to many overlapping curves, each with its own maximum intensity spike\s, and what we experience is the combination of these effects. In terms of waves, this means intensification or nullification based on how the waves superimpose on each other. Three big waves coincide and whammo, you have big symptoms. Which leads me to a question I have not thought about much, are there stages to PEM, with different symptoms in those stages?