"Why does the sun go to bed and then the moon comes instead. Then in the light of day, the sun chases the moon away?"

I have a secret that I shall tell, if I can be sure you would keep it well.

The sun NEVER goes to bed. She travels around the world instead. So in the place she used to be, she sends the moon so we can see.

So that is why in the light of day, the sun chases the moon away.