Apple Watch sleeping mode

The current Apple Watch has only 3 useful power states, OFF, Charging/Nightstand, ON. The Power Reserve / Low Power mode is of little value for your concept, because the only way to unset it is a reboot so you might as well just power down.

Your watch should go automatically to blank screen for battery saving when it is idle, and wake when you lift your arm or tap the screen. If that is not what you need please post back with a little more description of what you would like to happen.