I track sleep and energy levels with a galaxy watch and there's a strong correlation with how I feel on a given day. Sometimes it surprises me, and day where I wake up thinking I slept well but after an hour or two I'll realize my energy level isn't great, and sure enough I'll check the app and see it's low, with a warning about my sleeping HRV or resting heart rate being away from my norm.

It's not perfect - there are definitely days when it's wrong. But overall I have a target of keeping my sleep and energy scores in the 90s and it's helpful. I think the most important thing is to keep in mind that it's an imperfect measurements but it's still the best one most of us have for now.