I bought my forerunner 255 mainly for the overnight hrv readings. They’re a reasonable indicator of overall stress (physical and mental). I was taking spot readings first thing in the morning for a couple of months prior to that and found the data fairly unreliable, hence wanting overnight numbers.
Body battery seems a reasonable indication, but I have yet to fully trust it. Sometimes these numbers can be downright discouraging - particularly if you’re a bit excited the night before a big ride.
It's more a trend I take note. I've had two great nights sleep and a few poor nights last week. The data must be fairly accurate, it reports how I slept. This HRV will just bolster the overall body battery functionality. Hospital does.