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Sleepy Boos are medium-sized Boos that Mario encountered in Hotel Delfino during the events of Super Mario Sunshine. Sleepy Boos appear in the hotel's attic and cannot be passed or jumped over. Even F.L.U.D.D. does not affect them. Even though they are sleeping, contact with them will hurt Mario. The only way to defeat a Sleepy Boo is for Yoshi to eat it, thus clearing the way.
If Mario is riding on Yoshi's back and accidentally runs into a Sleepy Boo, he gets flung off the Yoshi, but if he attempts to get back on the Yoshi, he still gets flung off. It is nearly impossible to mount on the Yoshi again after he runs into a Sleepy Boo.