Heart Rate Variability - HRV (rMSSD)
While most of the HRV apps use some sort of “score” conversion, or adjust the score in relation to trends, as long as the rMSSD value is available, it should be comparable from app to app (even if the app doesn’t make the rMSSD transparent, it’s usually possible to figure out how they’re transforming it into a proprietary score). I’m not sure what apps/services have APIs or would be easy to partner with, but manual entry would at least be a starting place.
Popular apps are Elite HRV, SweetBeat HRV, BioForce HRV, Ithlete, and my personal favorite, HRV4Training (which seems to have the most integrations with other services).
Attribute now exists, supported by Oura integration initially.