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You might be interested in this https://www.59breaths.app/

Doesn’t let you measure HRV (so easily, as an application). If you mean using the Apple Watch as the sensor.

There’s a couple of HRV apps for iOS though.

You can get HRV data after a “mindfulness” session, but not in realtime during the session, at least as far as I know. That’s what I meant with not so easily.

I also found that the results don’t match measurements with a HR strap very well, at least for me.

The Apple Watch measured HRV when using the Breathe function in the Mindfulness app. Apps like Health, Training Today, and HRV Tracker can help visualize these readings.