Nocturnal animals are active during the night, and diurnal animals are active during the day. In addition, both sorts of animals are adapted to the differing light levels and challenges of the different parts of the day that they are awake.
Nocturnal animals are frequently vulnerable animals of prey. For example, mammals as an order were almost entirely nocturnal until their competitors and predators, the dinosaurs, became extinct. Small mammals, such as mice and tree shrews, remain mostly nocturnal in habit.
Nocturnal animals tend to have very large eyes. This increases their capacity to absorb light and produce an effective image on the retina. Some nocturnal animals have a membrane in their eyes that reflects light back into the retina for maximum absorption. This membrane, called a tapetum lucidum, is what produces the eerie glow in the eyes of a cat.
Their eyes are further adapted for low light levels by being packed with specialized retinal cells called rods. Rods are cylindrical cells that are very good at picking up contrast, light and shade, but they are unfortunately poor at color vision.
Diurnal animals tend to have better color vision because their eyes contain cells called cones. Different cone cells pick up different colors. Humans have cone cells specialized to pick up red and green and yellow and blue, which is why color blindness chiefly affects distinguishing between those colors and not others. Diurnal animals also have better visual acuity. Cones are better than rods at producing a sharp retinal image. Many diurnal animals, such as monkeys, most birds, bees and butterflies, require the greater visual acuity of cones to survive in the three dimensional world of the tree tops and sky, foraging for flowers, nuts and fruit.