Edit: Okay correction it seems like when I set a ship to Sleep it is not part of the idle ship list. (not that i want it to be, but it says the ship is idle in the description of Sleep, which is confusing). Meanwhile it still holds that when I set a ship to Guard it remains in the idle ships list which annoys me.

Edit Part Two: Okay was not realizing one can have both Sleep and Guard active at the same time setting the Fleet to Sleep while guarding will take it out of the of the idle ships list. Will make a suggestion post to change the tooltip of Sleep to better reflect that the Ship is taken out of the list of idle ships one cycles through when clicking the button at the end turn wheel.