The Sleep fleet action tooltip says : "Remain idle on this System until an enemy enters it or a probe is built."

I would suggest not using the word "idle" in the tooltip as the term is used to describe ships that have not been assigned an action. Namely in the function used to cycle through fleets that have no action assigned which says "Click to select the next idle fleet of your empire."

Edit: Should have been posted in the Game Design Subforum sorry for that.