You have seen night mode apps only on desktop that changes your monitor screen’s temperature to reduce eye strain. f.lux an app that is used to reduce computer display at night works on Windows, Mac, and Linux, is also available on Android, but requires root to work fully.
In Android 7.0 Nougat, Google built-in Night Mode feature and here is how to enable it.
Step 1: Download and install Night Mode Enabler from Google Play Store.
Step 2: Pull down on your notification bar twice to open the Quick Settings menu
Step 3: Press and hold the little gear icon for about 10 seconds.
Now open Night Mode Enable app and tap Enable night mode button. This will bring you to the System UI Tuner entry for Night Mode, which is hidden by default. Here you have to click On to enable Night mode. Once you’ve done this, Night Mode option should be included in your Quick Settings title along with Bluetooth, Airplane mode and few others.