hi @guyc, just a pointer here regarding our app, I am sure there are alternatives, but hopefully this is still useful information:
in HRV4Training we recommend measuring with Breathe in the morning. However, the app simply reads the last data point from Health (actually the RR intervals, so we can re-compute rMSSD instead of using SDNN which is not ideal). Hence normally you would use Breathe, then read the data immediately after, fill in the questionnaire, etc.
If you do not take a measurement with Breathe, most likely the watch still has some HRV data from the night, so you can just read it instead of measuring. This process still requires that you open the app and read the data when you wake up (or shortly after), as we look only at the past few hours. I would still do so in any case as the questionnaire and additional information is of great help for you to make sense of the data, as it’s all context that can help understand why there are certain physiological responses.
In general, I would recommend not using this approach, as for reasons I’ve tried to explain in various posts linked above, you’d end up measuring in an unknown sleep stage and time, both factors affecting the validity of the data (both the circadian rhythm and sleep stage might have a large effect, while if you measure in the morning, you remove both issues)
However, practically speaking, you should be able to use HRV4Training in this way to compute rMSSD from the last data point found in the app (just one data point as the use case is the one detailed earlier).
Hope this helps, some of these considerations are reported in these 4 slides I just put together: https://twitter.com/altini_marco/status/1349390092671049729