Two Exports, Different Data — Undocumented Changes to Apple HRV algorithm?

This is a very interesting paper by Jukka-Pekka Onnela, describing how exporting HRV data from an Apple watch at two different times delivered very different data sets. The guess is that the difference between the two exports is due to a change in the algorithm: “…these algorithms change over time, which can invalidate even within-person comparisons”