Righteous people, (among them the Alter Rebbe and Rabbi Hillel of Paritch) used to sleep before Shabbat, since it is written in books of Kabbala that that time is ‘Dormita d’Zeir Anpin’, (a time that supernal energies are resting near HaShem). Aside from that, just as it is appropriate to come into Shabbat not overly hungry from fasting, it is appropriate to come into Shabbat with a clear and steady mind.
