https://pine64.org/devices/pinetime/
Be warned though, the hearth rate monitor doesn’t work particularly well. And there is no sleep tracking afaik.
If you’d prefer something more reliable (but less open), GadgetBridge is an Android app to interface with commercial smart watches through reverse-engineered protocols.
Do you happen to own one? If yes, how do you feel about it?
For example, in the PineTime there is a hearth rate monitor, but it’s too slow and imprecise. Notifications work great, and the battery lasts 20 days or more. How about the Bangle.js 2?