Q. When Shabbos is Erev Shavuos can you sleep on the Shabbos afternoon if you don’t normally sleep then or sleep less, or is this a prohibited hachone (preparing) from Shabbos to Yom Tov?

A. Although in principle preparing on Shabbos for after Shabbos is permitted when done for a mitzva purpose (Shulchan Aruch O.H. 306: 3,) not all hachanos are permitted, (Shaarei Teshuva 693, in regards to bringing a megila for the night, Beis Shmuel, E.H. 169: 5 on dedicating on Shabbos a place for chalitza.) Sefer Chasidim (266) teaches that a person should not say, I will sleep now on Shabbos, so I will be able to write chidushei Torah on Motzei Shabbos, since it denigrates the honor of Shabbos.
Horav Shlomo Miller Shlit’a added that he may end up not be learning all that much on Shavuos night after all, and he could have learned better on Shabbos itself. However, in principle the Rov permits sleeping on Shabbos as he needs, as long as he does not specific mention that he is doing it for preparing for the night, since after all he does benefit and enjoy the rest on Shabbos.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a