A Conjuration spell that requires 4N, I would assume.
Open up the game, look at the list of spells in the lab under conjuration, filter on Nature (press 'N') and look down the list for something that mentions Sleeper.
Well, thanks for tip about pressing the "N".
So "Awaken Sleeper", lvl 5, not so easy to find in the index, my apologies.
Sorry if it got your back up...