Whereas taking a snooze on Shabbat afternoon is a mitzvah, there is a strong custom not to sleep during the day on Rosh Hashanah.
As the rabbis say, when the books of life and death lie open, who can sleep? The awesome stakes on Rosh Hashanah - the state of our lives - call for heightened awareness and wakefulness.
The 16th-century mystic Rabbi Chaim Vital reports that it was the custom of his rabbi, the Ari (Rabbi Isaac Luria) to rise at dawn on Rosh Hashanah and pray at sunrise, allowing himself to sleep in the afternoon.
The late Sephardi Chief Rabbi of Israel, Rabbi Ovadia Yosef, added that sitting around, doing nothing and wasting time on Rosh Hashanah is like sleeping.
In fact, Rav Ovadia says that it is better to go to sleep than to spend Rosh Hashanah that way.