When does the narrator suggest is the best time to visit zoos? Why?

From the text:

In zoos, as in nature, the best times to visit are sunrise and sunset. That is when most animals come to life. They stir and leave their shelter and tiptoe to the water's edge. They show their raiments. They sing their songs. They turn to each other and perform their rites. The reward for the watching eye and the listening ear is great.