Sforno on Exodus 31:17/ספורנו על שמות ל״א:י״ז

1וביום השביעי שבת, seeing that on that day the creative activity of G’d had been completed, completion spells rest,

2וינפש, this is why the seventh day is one devoted to the נפש, the spirit, resulting in G’d giving Jews an additional soul for use on that day. This additional soul assists us in concentrating on the spiritual dimension of the day. When man was created, G’d made him in His image, i.e. first and foremost concerned with spiritual concerns. The extra soul granted us on the Sabbath is to help us live up to that vision G’d had of man when He created him. (compare Beytzah 16)