As I got older, my morning resting and orthostatic HRs became less reliable indicators of training state, so I added morning HRM about 2000 days ago, taken both with a phone app and my Polar watch. For sure, if your HRV drops and keeps dropping and then stays down, something's wrong. That's what clued me to see a doctor and discover that I had PMR. I make my phone HRV more informative by doing 3 minutes lying down and then 5 minutes standing. A drop in my standing HF power is my first clue that something's up, usually nothing, just variations in training response, but sometimes it's been something.
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Results matter