[StavrosK]

True, I guess the watch must have some sort of external circuitry that can wake it up every so often.

[sokoloff]

You can use the ESP32 onboard RTC to do periodic wakeups. (You can also use external stimuli via GPIO pins, but you don't need anything extra if all you want is a periodic wakeup.)

[StavrosK]

It has an onboard RTC? Very nice, I missed that (and the ESP8266 didn't have one).