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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.
user157504
yes, it is off even when rebooting, only when you hold button to reboot and then release in the middle while the bar going past the screen and it says "DFU start" then it is advertising and waiting for firmware update
When it is not advertising (=something you call "just turn off discovery or discoverable, or some low power option") there is no transmission at all. It is not some low power mode, the radio part is simply completely off. Well, unless your watch actively runs some javascript code to search for other devices and/or is trying to connect to them.
When it is advertising it is still very very far from something that could be called "bluetooth signal near my head for long periods of time" and also the power of the signal is very low. That is why your watch lasts weeks on tiny battery (and BLE beacons last about a year on single CR2032 battery). Unless you turn off your phone completely during night and also turn off your wifi router I think it makes no sense to worry about this.
The BLE advertising (of bangle.js) is sending 3 packets each taking about 500 microseconds once per 380 miliseconds. So about 378.5/380 of the time the radio is not transmitting = completely off. And the radiated power when transmitting is about 2.5mW (wifi router is about 100mW)