Are you on the normal 2v13 firmware? This was an issue with the latest bangles and got fixed a few weeks ago so if you try a 'cutting edge' build it should work fine and start the HRM as soon as it detects skin nearby.
Well that would explain it... updating to 'cutting edge' now. I wrote a callback function to 'restart' the PPG sensor (stop(),start()) after a handful of zero/NaN HR values and to only restart when accelerometer detected motion ... no point wasting battery if the watch is sitting on the table.
Espruino is a JavaScript interpreter for low-power Microcontrollers. This site is both a support community for Espruino and a place to share what you are working on.
Are you on the normal 2v13 firmware? This was an issue with the latest bangles and got fixed a few weeks ago so if you try a 'cutting edge' build it should work fine and start the HRM as soon as it detects skin nearby.