Ask HN: Hackable wearable with HR monitor?

Is there any wearable currently available that provides:

  - All-day heart rate monitoring
  - Step counter and sleep tracking
  - Access to all relevant raw data?

I currently own a Fitbit charge 2 which is terrible - the heart rate monitor is not even close and sleep tracking is wrong. I want to access the raw data so I can implement my own algorithms for e.g. sleep tracking, activity level, etc. Does anyone have experience doing this?