There’s a setting on my Garmin watch which reports my energy levels. Over the last few weeks they’ve gone from medium to low, to acutely low, to almost nothing.
I don’t know how it works it out – I guess it’s from sleep data, steps, heart rate, Sp02,etc.
Interestingly I don’t feel like I’ve been out of energy. It’s just been directed in ways which the Garmin wouldn’t recognise – reading, writing, audio editing and recording, design, etc.
If you are using a yardstick to measure what happens, don’t make it so specific it can’t see the wood for the trees.