Nyctalopia refers to night blindness or difficulty of the eye in visualizing under dim light or at night. Daytime vision, however, is unimpaired. Nyctalopia is due to the eye's inability to adapt quickly from lightness to darkness. The principle cell-type associated with Nyctalopia is rod cells. Rods are a type of photoreceptor cell present in the retina that transmits low-light vision and is most responsible for the neural transmission of nighttime sight. Rods have a singular photopigment, rhodopsin, which utilizes the protein scotopsin and the Vitamin A-derived cofactor, retinol. This cascade is essential for the body's ability to regulate the pupillary light reflex. The pupillary light reflex allows unilateral afferent detection of changes in light energy entering the eye, and efferent adjustments in the pupillary sphincter and dilator pupillae muscles to initiate consensual constriction and dilation of the eyes. Pupil dilation is an adaptive response to changes in lightness and darkness. Night blindness is the physical manifestation of impaired functioning of these processes.