Active calories while doing nothing ?? : Apple watch

I find my watch that logs active calories while doing nothing. I wake up in the morning and there will be 10-15 calories burned little by little during the night. What confuses me is my heart rate kept between 43 and 50 beats per night all night. No sleep.

Same with a nap afternoon. Same with sitting at my desk and working.

I routinely go through my active calorie data and clean out 25-50 calories daily – usually deleting all entries that are less than 0.25 calories during known periods of inactivity – to try to make up for this behavior.

Is this normal? And what does it think about tracking, exactly?

(I've calibrated my watch with the long walk / run-in-the-open-to-20 minutes method).