The search for the Castle continued until around three in the morning, with the inevitable result that nothing was found, as though Black had never broken into Hogwarts Castle at all.
Professor Dumbledore must have anticipated this outcome, but he still had to have the professors search the entire Castle to reassure the students and staff, and to provide some accountability to the Dementors stationed outside the school grounds.
This, however, was rough on the professors and Prefects conducting the search, who were unable to close their eyes all night. Especially when fighting off exhaustion after a full meal, trying to stay awake was a genuinely difficult task.
Chapter 636