What causes pupil constriction?

Pupils constrict to regulate the amount of light entering the eye. This action of the eye is known as the pupillary light response or pupillary light reflex, according to Science Daily. It is a normal response controlled by the optic and oculomotor nerves of the eye.

Pupils also have the ability to dilate in response to the amount of light entering the eye. The constriction of the pupil helps decrease excessive light which can be damaging to cells in the retina. WebMD reports that the retina is a layer of the eyes laden with photoreceptor cells integral in forming images and the perception of objects.