People who are night-blind have trouble adapting to diminished light. For them,l the normal chemical reaction that allows the rods and cones to adjust to the amount of light is either absent or inhibited. In some cases the cause is a vitamin A deficiency. But, alas, eating carrots won't help everyone. Other causes include disease, heredity, and prolonged exposure to bright light.
And even when vitamin A might be the answer, it's possible to have too much of a good thing. Taken in great quantities (such as high-strength vitamin A pills), it can cause blurred vision, stomach and skin problems, headaches, and even brain damage. One sign that you're getting too much vitamin A is a yellowish tinge to your skin, due to the pigments found in vitamin A. As a rule of thumb, one large carrot every two days ought to be enough to counter any deficiency.