Night blindness is impaired vision in dim light or darkness, one of the earliest clinical signs of vitamin A deficiency. The retina’s rod cells depend on rhodopsin, a light-sensitive pigment built from retinal, a vitamin A derivative; when vitamin A runs short, rhodopsin cannot regenerate fast enough after bleaching by light, so adaptation to darkness fails. It is a major public health indicator in regions where deficiency is endemic, often appearing in preschool children and pregnant women before the more severe eye damage of xerophthalmia. The condition usually reverses quickly with vitamin A supplementation. Beyond nutrition, night blindness can also result from retinitis pigmentosa, cataracts, or certain medications, so clinicians distinguish nutritional from structural causes before treating.