How do I set night light (or night mode) to turn on automatically?

With the screen on, access the "Quick Settings" menu by swiping  the screen from top to bottom. Swipe down once more to display more icons from the Quick Settings menu.

Tap and hold the Night Light icon (looks like a crescent moon). Tap on "Schedule" and select from different options. For custom times, select "Turns on at custom time" and then tap on the "Start time" and "End time" below to establish the preferred times.

Note: the Start time is typically in the evening, say around 8pm. The End time is typically in the morning, say around 7am.