You can use the night shift function to read the screen when the room is dark. It is more smoothing for your eyes.

Go to Settings – Display & Brightness – Night Shift to enable the function. You can adjust the warm tones according to your needs.

Reminder
It is not recommended to use your phone in a dark environment for a long period of time. It will strained your eyes. The eyes will be wet and tears will drip if you use over a period of time.