Yoma 1:6-7

Yoma 1:6

The Kohein Gadol was not permitted to sleep on the night before Yom Kippur, so what did he do? If he was learned, he would teach Torah. If not, Sages would lecture in front of him. If he was proficient in Scriptures, he would read from them. If not, they would read to him. The selections read were from Job, Ezra and Chronicles. Zechariah ben K’vutal said that he read to the Kohein Gadol on many occasions from the Book of Daniel.

Yoma 1:7

If he started to nod off, the young kohanim would snap their fingers and say, “Sir High Priest! Arise and drive sleep away once you are on the floor!” (The floor was marble and the kohanim were barefoot, so the cold floor would wake the Kohein Gadol.) They would occupy him until the time to offer the sacrifices arrived.