The young lions roar after their prey, and seek their meat from God.
 The sun ariseth, they gather themselves together, and lay them down in their dens.
 Man goeth forth unto his work and to his labour until the evening.
Remember, this Psalm parallels the creation story of Genesis 1. On the fourth day of creation the sun and moon were made by God. God controls the darkness and the light along with the life cycles of night and day creatures. The Psalmist is simply declaring that God designed us to appreciate the concept of daytime and nighttime.
It could be noted that most meat-eating animals (carnivores) do their hunting at night when their prey is less likely to see them coming. The idea is that God designed daytime for most of us to "live our lives" when such dangerous animals are sleeping. Among the reasons God created “day and night” is so there is a time of separation when such animals hunt and the time of the day where most people should do our work.