The brighter your screen, the easier it is to read… and the more battery power it consumes. To conserve power, you should experiment with brightness and set it to the lowest level you can comfortably work with.
To set the brightness level, go to the Settings >Brightness window and move the sliding bar to increase or decrease it. You can also activate Auto-Brightness from this window. When turned on, your iOS device uses the built-in ambient light sensor to adjust the screen brightness for current light conditions.