We are used to seeing animals in the daylight. That is because our eyes are adapted to seeing best when there is lots of light about.
But other animals are different. They have eyes and other senses adapted to work best when there is very little light.
This owl and the mouse are examples. Other nocturnal animals you may know of include rabbits, hedgehogs, bats and many kinds of moth.
The world does not go to sleep at night. It is simply that a whole new range of animals take over.