It is hard to see in the dark immediately after you turn out the lights, just as it can take a few minutes for your eyes to adjust when you walk from a dark place into a bright, sunny day. Why? Your eyes’ pupils are responsible. The size of your pupil the black circle in the center of your eye controlled by the iris determines the amount of light that enters the eye and adjusts to light and darkness. Pupils dilate (widen) in dim light or darkness to let more light into the eyes; they constrict (narrow) in bright light to prevent too much light from entering the eyes. When you move from dark to light, pupils widen, and the retinas become six times more sensitive after one minute in darkness than they are after the first few seconds. After a half hour, they are even more sensitive, and you can begin to see more comfortably in the dark.