If you are using your Android device as an alarm clock for waking up in the morning then you would probably want the display active throughout the night if you need to check the time, which is why Night mode is so useful. Rather than have your Android device go to sleep with you and have absolutely nothing displayed on the screen, you can tap
the Menu icon (three dots) in the lower-right corner of the main clock screen and select Night mode from the menu.
Once the latter is activated, the clock will then act as a kind of screensaver for your device and be ever-present throughout the night so you can always check the time if you happen to wake up. To quit out of Night mode at any time, simply tap the screen and the regular Clock app display will appear.