In Overture #6, the final issue of the Sandman's prequels, Morpheus expends a considerable amount of energy recreating the universe by using the dreams of the beings on his vessel:
This leaves him vulnerable, which is why he is captured and imprisoned in The Sandman
This should immediately remind you of the story in Dream Country, where a thousand humans are able to rewrite the history of an entire universe by have the same dream:
Yet in this story, there is no mention of Morpheus (or the cat version of him) getting tired. It seems a bit silly that he would ever allow such a thing to occur if he became so weakened.
So why is there a difference? Why was Morpheus not noticeably tired when changing the universe in The Sandman, but was exhausted in Overture?