One morning you wake up as tired as when you went to sleep. You feel worried, relationships suffer. But the next morning, after a good night rest, you feel fresh and alert. The circumstances of the previous day may remain the same, but your evaluation of them differs dramatically. You are more relaxed, yet more energetic, more productive.
Relationships are smoother, more harmonious.
Basically, it's because the world is as you are. Put on green glasses and everything appears green. Put on yellow glasses and everything is yellow. Look through tired eyes with an anxious mind and your vision is clouded with problems, many of which, in reality, may not exist. Look through fresh eyes with an alert, creative mind and you are better able to see solutions to the problems that do exist.