Nyctalopia (literally "night blindness") is a condition making it difficult or impossible to see in the dark. It is a symptom of several eye diseases. Night blindness may exist from birth, or be caused by injury or malnutrition (for example, a lack of vitamin A). Carrots are known to to improve eyesight because of their high vitamin A content.
The commonest cause of nyctalopia is retinitis pigmentosa a disorder in which the rods in the retina gradually lose their ability to respond to the light. Patients suffering from this genetic condition have progressive nyctalopia and eventually their day-time vision may also be affected. In congenital stationary night blindness the rods do not work from birth, but as the name implies, sufferers do not get worse.
The opposite problem, known as hemeralopia, is much rarer.