Yoma, Chapter One, Mishnah Seven
This mishnah shows how the young priests would keep the high priest awake all night on the eve before Yom Kippur.
1) If he wished to sleep, young priests would snap their middle finger before him and say: Sir high priest, stand up and drive the sleep away by standing once on this [cold] floor.
2) They would keep him busy until the time for the slaughtering [of the daily morning offering] would arrive.
Section one: There were two things that the young priests, those with the strength to stay up all night, would do to keep the high priest awake. First they would snap their fingers at him and then they would tell him to stand on the cold floor. The rabbis believed that standing on a cold floor would wake up a tired person. If youre not sure, try this yourself.
Section two: Finally, they made sure to keep him busy all night so that he wouldnt fall asleep.