Sleep Tracking Grey Heartrate Bars

I tried out the new sleep tracking ability last night for the first time, which I’m excited about using regularly. However, I was looking at the heart rate readings this morning and couldn’t figure out what the grayed out sections of some of the bars are supposed to mean. If there was no reading at those levels at that point in time, wouldn’t it just be blank? Does the grey bar mean there was a partial reading of some kind, but with not enough confidence to be a red bar? If anyone knows the answer, I would appreciate it— I tried to find an answer online and couldn’t. I’m going to be looking at these analytics a lot in the future, and I’d like to know exactly what they mean.

See the included picture to know what I’m talking about.