I've been running this a bit with a Polar chest strap heart monitor. I'm thinking about forking it to add some audio cues so that I can have it running in the background while I work and try to keep my heart variability up. I find that I tense up when I'm intent on the work which leads to a lot of problems. I'm hoping having something like this app that uses audio cues for breathing but only comes on when my heart variability drops into the red could get me into a continuous state of low sympathetic nervous system activation while working, which is very much not the norm for me for historical reasons.
The author of this tool eventually created a heart rate monitoring hardware product and an app to go with it to do HRV training. I think `every-breath-you-take` may have been an early prototype that he generously open sourced(?)