Sleepy Sheep is an item in the first three Paper Mario games. When used, many sheep will run across the screen, causing most enemies to fall asleep. Some enemies have a higher resistance to sleep than others, so it is never guaranteed that this item will affect all enemies in a battle.
In Super Paper Mario, Sleepy Sheep can sometimes be dropped by enemies, and if given to Saffron, she will cook a different Mistake that causes sleep.