You must think it's normal to be awake during the day and sleep at night, but that's not true for all animals. Humans are diurnal, the scientific term for tending to be up and about during the hours when the sun is up. Being awake in daytime has some obvious advantages: It's warm, it's easy to see, and at least for humans, sunshine provides much-needed vitamin D and also makes us feel good. But for many animals, being up at night, or being nocturnal, is a great arrangement. Small animals who are in danger from predators can gather food more safely in the dark. And some animals can fix their way around by using senses other than sight - bats, for example, use their sense of hearing to navigate. In hot climates, it can also be a good thing to avoid the bright sun and spend the day curled up in a cozy cave or den.

A. Sunshine provides much-needed vitamin D and also makes us feel good
B. Humans are diurnal
C. It can also be a good thing to avoid the bright sun
D. It's normal to be awake during the day and sleep at night