I haven't seen Withings Sleep mentioned - https://www.withings.com/us/en/sleep .

This one is basically "set it and forget it". You put it under your mattress and calibrate it, then it tracks automatically.

I like it because I don't have to bring any gadgets into the bedroom, or remember to turn on tracking.

It seems pretty accurate from what I've seen so far, although I don't have anything to compare it to. I've found it very useful for testing various sleep interventions.