====== Sleep Modes ======
  * [[https://stackoverflow.com/questions/52767240/light-sleep-with-gpio-wakeup-in-esp8266|Light Sleep with GPIO Wakeup in ESP8266]]
  * [[https://github.com/esp8266/Arduino/blob/master/libraries/ESP8266WiFi/src/ESP8266WiFiGeneric.cpp]]
  * [[https://github.com/espressif/ESP8266_NONOS_SDK/issues/165]]