Falling asleep in a dungeon is just about the most irresponsible thing you could do. Aren't there treasures to be found? Dungeons to be delved? Wraiths to curse at? Shame on you!
Most likely though this was the result of a consumable so you may have only your curiosity to blame.
Or maybe you got a little bit too close with a Nymph?
Simply wait until you finish your nap and the Zs above your head go away.
Falling asleep in the midst of battle from your own hand or from a Nymph is a dangerous position to be in. You will be unable to manage your inventory or move in any way until you wake up.