Every living organism depends on sunlight for its survival, in one form or the other. All of us benefit from the Sun's effect on our bodies, because sunlight on skin produces vitamin D, which is important in the formation of strong bones.
Sunlight also affects the behavior of animals in different ways. Migratory birds know that it is time to fly to warmer places when the hours of daylight become shorter. Some mammals also know that it is time to hibernate as the days become shorter and shorter, Spring, when the hours of daylight start increasing after a long winter, is a time for courtship for many birds and animals. Similarly, hens need a certain amount of daylight in order to maintain peak egg- laying patterns. It is believed that animals can sense changes in the quality of light, and its duration with a part of the brain called the pineal gland.
A reptile found only in New Zealand, the tuatara has a third eye, called pineal eye, located on top of its head that is light- sensitive and controls the behavior of the animal.