I previously recommended the Oura Ring 4, I'm happy to update this to the new generation 5 ring.
It's the wearable that Apple should have made years ago. No screen, no buttons, no on/off, just well-presented data about your sleep, movement, heart rate and HRV. Battery life lasts a solid 5 days, and the battery case provides a good 4-5 full charges – enough that I rarely ever have to think about it.
Don't skip the sizing kit – it worth getting the right size the first time.