During the day, less light is needed to see the things around you. At night, however, more light is usually needed to see things you might have seen easily during the day. To control the amount of light entering the eyes, the irises widen or narrow to change the size of the pupils. For example, when it's dark, the irises widen, and the pupils dilate or get bigger to allow as much light as possible to enter the eye.

Source: Wonderopolis