7 reasons why your pupils may be dilated

Lighting conditions
Just as a camera requires the correct amount of light to capture vivid images - your eyes also need the correct amount of light to see properly. During the day less light is needed to see things around you, which is why the iris narrows and the pupil constricts and gets visibly smaller. Conversely, in darker conditions, the pupil will enlarge as your eye will try to let in more light to see things you otherwise would have easily seen during the daytime.