Hi All... new poster here! Bit of background about me: I'm male and 32 years old. I've suffered from M.E. since I was 18, after getting glandular fever and mumps in the same week at college. I was pretty bad until about 27 when my condition seemed to improve and I could start exercising again. I would now say that I am relatively okay. I go through periods of being fine and then set-backs (usually triggered by a virus). I started running again at age 28 and have since become fairly proficient. I can do 5k in 17 minutes and 10k in 36 minutes. In all the time I've been running I've always had a Garmin watch with built in Heart rate logging, or worn a HR strap if i want to ensure higher accuracy particularly on fast track sessions. Its amazing the amount of data you can amass just from the beat of your heart. I'm so familiar with it that I could probably look at a trace and tell you exactly when I'd had caffeine, climbed steps, had a cold, when it was a hot day etc.. Now, over this time I've also started seeing patterns and strange things happen when I have bad M.E. days. (Just to add: I do exercise on my bad days too, fighting fire with fire seems to work for me in getting over relapses) Here's some examples of weirdness: My HR on a half marathon race. This is very normal... HR rises steadily as the race goes on. I felt great this day and ran very well. Screenshot_20190511-125954.png Here's a few weeks ago on an 8km run when I felt terrible. Look at all the crazy spikes and troughs... pretty much mirrors how i feel with a very fluttery and jumpy heart. Screenshot_20190509-030334~4.png This is my HR taken just after i wake up. This is me feeling good... very flat and steady in my sleep with the odd spike (probably when i move in my sleep) Rest Normal.jpg Here is after I wake up on a bad day. Notice how spiky my HR is in my sleep and how it gets way higher even before the point i just wake up. I've correlated this enough with bad days to know it is related. Rest ME.jpg There was one time i had terrible Dyspnea, but I decided to go to track training anyway. We did fast intervals of 400m and 800m. I literally could not grab any air and started to get seriously light headed.. honestly thought i was going to pass out. Normally in such a session, My HR during those hard efforts should be pretty high - getting up to 170 or 175. Look at the trace from this session - its almost like i've maxed out at like 135 and i cant get any higher. FYI my heart was really really sore for weeks after this, like I had strained it Dys.png That's just some of the oddities i've found anyway, maybe someone can use this data or use it to verify their own findings. Please feel free to ask any more questions!