I am just coming out of a 'crash' and have been thinking about what could explain PEM... This last bout was I think brought on by socialising, possibly on the back of too much physical activity preparing for Xmas... But what is it that can cause all systems to slide so quickly? I feel not only the physical fatigue, but also huge cognitive fatigue, and also what I can only describe as a hormonal wave... Sinking down. It's like watching the waves on a beach, and suddenly realising that the sea has retreated too far and a tsunami is coming. Is energy production so similar throughout the brain and body that a problem with this can cause all the symptoms? And so quickly? Is there really the fluctuation in hormones/neurotransmitters that I feel? Sorry if these are daft questions, but I see PEM discussed lots in terms of what we do to bring it on, and what it feels like, but not why it happens at a biochemical level.