What this measures
The small differences in time between one heartbeat and the next while you were asleep, in milliseconds. It is a measure of variation rather than of speed, so a higher figure is not a faster heart.
How your device works it out
Your watch or ring samples your pulse through the night, keeps the gaps between beats, and reduces one stretch of the night to a single figure. Providers pick different stretches and different maths, which is why two devices on the same night rarely agree on the number.
What generally moves it
Sleep timing, alcohol, a late heavy meal, heat, hard exercise the day before, travel across time zones, and how well the band or ring sat against your skin. Age and fitness set roughly where your own figures sit, which is why yours are only worth reading against your own.
Why Thriverence shows it
It is the input Readiness leans on most, read as a trend against your own usual range rather than against anybody else. That is also why the screen shows a band built from your own nights instead of a target to hit.
What it is not
This is not a measure of health and it does not detect anything. A low night is not a warning, a high night is not a clean bill, and no reading here is evidence of a medical condition. If something about how you feel is worrying you, the resources below are the way to reach a person.