Charge 4 won't track my heart rate accurately at night

[PerfectBlue]

My charge 4 functions perfectly during the day, absolutely flawlessly, but at night I'm getting these big gaps in my heart rate record.

Usually 30 minutes to an hour.

The sleep record continues but there are just blank spaces in the heart record.

Usually these seem to start if get out of bed for some reason, but others are completely random. 

I haven't changed any settings and I'm not wearing it any differently.

This happens 2 3 times a night and never during the day.

I've been reducing my screen time before bed and cutting down on cafine and sugar so my resting heart beat has dropped significantly. Is it possible that my heart beat got too low to register properly and it thought I'd taken it off?

I've tried cleaning the sensor and checking my band fit and position. 

Moderator Edit: Clarified subject

[LZeeW]

@PerfectBlue  I've always felt that any gaps in sleeping heart rate come from an arm position that interferes with the LED, like your arm tucked under the pillow.  Your observation that this doesn't happen during the day suggests that this might be the case.  As far as a sleeping heart rate being too low, Fitbit used to have a Help article that listed the heart rate accuracy range.  I remember reading that the heart rate was accurate to 30 bpm while sleeping.  I have a low RHR and a lower sleeping heart rate.  I'm not using a Charge 4 right now, but I often having sleeping heart rates in the high 30s.  I don't get gaps.