What is UVA light?
UVA (400-315 nm) makes up about 95% of the UV radiation that reaches the surface of the earth. Humans cannot see UVA, but many species of reptiles can. While humans have 3 types of cones in their eyes that allow for color vision, reptiles have 4 types of cones, with the 4th cone detecting UVA. Many reptiles also can detect UVA with their pineal gland. Therefore, it should come as no surprise that reptiles denied UVA tend show less interest in food and an overall reduction in activity.
To visualize how a lack of UVA would affect your pet reptile, imagine yourself going through life colorblind. Think of all the vivid colors you would miss out on. Many brightly colored foods would be much less appetizing. You'd have a reduced perception of the environment around you, and your overall well being would be greatly diminished. UVA should be provided for all diurnal reptiles when possible.