The night mode is also available on Android 9 for Samsung devices running One UI. The night mode changes to a darker theme to protect the eyes.
On devices with an OLED display, the battery consumption is also greatly reduced. The night mode can be turned on/off either manually or by a schedule.

Android 10 | One UI 2.0
Night mode -  Open the  Settings
Night mode -  Tap on  Display
Night mode -  Enable or disable  Night mode
  1. Open the Settings
  2. Tap on Display
  3. Enable or disable Night mode

Samsung Instructions