Or HaChaim on Exodus 20:11/אור החיים על שמות כ׳:י״א

1 וינח ביום השביעי, He rested on the seventh day, etc. The Torah cleverly forbids among the 39 categories of forbidden work also such activities which do not involve something causing fatigue. Moving an object from a private domain to a public domain or vice versa does not represent any "work" in the regular sense of the word. There are other such activities (writing two letters of the alphabet for instance) all of which are nonetheless prohibited on pain of death. A person may say to himself that since such acitivities do not involve physical effort why should there be a death penalty for performing such activities on the Sabbath? The Torah answers that the work prohibition is not related to the amount of effort involved. If G'd rested on the seventh day it was certainly not because He had become tired of creating the universe and needed a rest. Isaiah 40,28 puts it succinctly: "He never grows faint or weary." The word מנוחה in the sense of rest as we use it, i.e. rest in order to recover from exhaustion, is quite inappropriate when applied to G'd. Any activity to which the term מלאכה is applicable is prohibited regardless of the physical or mental effort involved.

2The words כי ששת ימים עשה also mean that G'd created the world in order for it to endure for six days. It follows that He has to "renew" creation i.e. to issue a directive for the universe's continued existence every single day. The means He uses is the soul of the day called Sabbath which He created by "resting." The fact that G'd desisted from creative activity after the sixth day resulted in the Sabbath coming into existence. Our sages in Bereshit Rabbah 10,9 indicate that prior to the Sabbath the world was in a constant state of turmoil. When the Sabbath came the world calmed down. Compare also what I have written on Genesis 2,2.