Every morning I check my heart rate on my fitbit tracker/app on my iPhone. Yesterday morning I found it as low as 38 BPM. Is that normal?

It only happened once before and was wondering if a resting heart rate should go down so low. Normally my resting heart rate can go down to 50 during a nights sleep. But 38 BPM seems too low.

Anyone know anything about this sort of thing?
Update: by the way I'm soon to be 63 years old
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