Doze Mode
If the watch is left idle for about 30 minutes after you remove it from your wrist it will enter a Doze mode that performs Android Wear operations only intermittently in order to conserve battery power. In Doze mode, the watch displays the current time using a monochrome LCD screen and minimizes communication with your smartphone.
To recover from Doze mode to normal operation, tap the watch’s touch screen or press its power button.