Yoma, Chapter One, Mishnah Four
This mishnah teaches that they didnt let the High Priest drink or eat a lot on the eve of Yom Hakippurim because they thought that eating a big meal would make him sleepy and they didnt want him to sleep that night. The reason that he wasnt allowed to sleep was lest he have a nocturnal emission which would make him impure and thereby prevent him from serving in the Temple the next day. Since seminal emissions cause one to be impure for only one day, they only needed to prevent him from eating on the eve of Yom Kippur. During the rest of the week he was allowed to eat and drink in a normal fashion.
All seven days they did not withhold food or drink from him. On the eve of Yom HaKippurim near nightfall they would not let him eat much because food brings about sleep.