Yoma, Chapter 1, Mishnah 4. One of the central concerns in preparing the Kohen Gadol for Yom Kippur is that he might become tamei through a seminal emission. Contact with a dead creeping creature can be avoided through care and vigilance, but something that occurs within a person's own body is far harder to prevent. For this reason they sought to keep him from sleeping at all on the night of Yom Kippur.
The words of the Mishnah:
"Kol shivas hayamim lo hayu monin mimenu maachal umishteh. Erev Yom HaKippurim im chasheichah lo hayu manichin oso le'echol harbeh, mipnei shehamaachal meivi es hasheinah"
"Kol shivas hayamim" - During the seven days of preparation preceding Yom Kippur, they did not withhold food or drink from him, and he ate as he wished.
"Erev Yom HaKippurim im chasheichah" - At the final meal before the fast they did not allow him to eat a great deal, since eating a large amount tends to bring on sleep, and sleep can lead to a seminal emission.
In addition, there are certain kinds of foods listed by Chazal as causing a seminal emission, and they refrained from feeding him those as well.