Apple Watch is not so “long-playing” gadget. Apple knows about it and have created a special power-saving mode named Power Reserve (PR).
Power-saving mode disables all functions of Watch except two – creation of screenshots and display of time.
How to enable power-saving mode
1. In a mode of display of time. Swipe upwards from the bottom of the screen, select Power tab, press Power Reserve, and then press Proceed.
2. Press and hold the side button on case of the Watch until Power-menu appears. Swipe over Power Reserve to enable the mode.
How to disable power-saving mode
The same, as in item 2 above – press and hold the side button until Apple logo appears on the screen. Watch will restart and enter a “normal” mode. Do not forget that the disabling of Power Reserve mode is possible only if the battery Watch was charged again.