Here's a different perspective I've noticed. While just relaxed normal body movement or at rest, sometime I've gotten a reading of 44bpm. I think I've noticed this happens occasionally when I touch the Heart app in Glances To trigger a reading. The next recording will be 88bpm, so doubled. I've noticed this pattern has happened this way on numerous occasions over what a year or so of wearing the watch.
I'm guessing the sensor to check the rate gets activated mid-cycle but that's just a guess. It's such a low number that it doesn't make sense. I'm laying down now on the bed and raised my wrist to do the check (holding arm steady) in Glances and this time it came up 42bpm then next reading was 88bpm. When I've looked back at my Show All Data individual dates, the very low rate and the next "normal" rate occur at the same time mark. So I just think it's interrupted when it gets triggered to provide a measurement. I never see consecutive recordings at these low marks.