I have noticed over the last year that should I exert myself a bit too much (usually by just trying to increase my activity level slightly), that when I try to go to bed, I feel uptight and not restful. It takes a long time to get to sleep and I will wake up several times, but once in a blue moon (Last night Included), I will wake up with a very tight chest, strong heartbeat and a feeling of tension throughout my body. It is as if my nervous system is in a panic or threat mode after such exertion, or that with the lack of energy in my body, all my cells are screaming for more energy/oxygen and that switches on a flight response. It has taken about six hours to calm my body down this morning and release the tightness in my chest, even with deep meditative breathing (which seemed to actually make things worse). It would be very easy for someone (a doctor) to say that this is anxiety related but I saw a good friend last night, had a calm evening and went to bed feeling content, and besides I haven't suffered from anxiety for well over a year now. For me, this is very physical and it only occurs when I am over-exerted. Presuming that it was to do with Mitochondria and Energy, the first thing I did this morning was to have a Mito supplement drink and I am starting to feel much better. I don't believe this a is a placebo. Has anyone experienced this horrible feeling or have any clues on the mechanisms behind it?