The Rama (O.H. 290:1) writes that if one is accustomed to napping in the afternoon during the week, one should not forego napping on Shabbat as it is considered a Shabbat-enhancing pleasure. The Arizal (Sha’ar Hakavanot) writes that napping is detrimental during the week but is very beneficial in rectifying the Neshama on Shabbat, especially for the righteous. Indeed, the Arizal had the practice of napping up to three hours on Shabbat. The HIDA (Birke Yosef, ibid) decries those who steadfastly uphold this custom the Arizal but not the others.
The Mishna Berura (ibid., MB:3) adds that although one should delight in napping on Shabbat, one should not spend so much time napping so as to lose out on learning Torah. Indeed, he continues, the Zohar (Bamidbar, 82b) writes that Shabbat is a time when one should spend time developing Torah novellae. The Ben Ish Hai (Mekabtziel), citing the Zohar, adds that when one comes up with a novel Torah thought, Hashem gathers all of the different spiritual communities and teaches it thought to them.
The Shulhan Aruch (ibid:2) goes on to rule that after the daytime Shabbat meal, the congregants should return to the synagogue or Bet Midrash in order to study the Nevi’im (Prophets) and Aggada (exegesis). Rabbi Yehuda Ayash (Mateh Yehuda, ibid:2), the HIDA (Mahzik Beracha, ibid:3) as well Rabbi Baruch Toledano Kitzur Shulhan Aruch ) all write that it is incumbent upon the rabbi of the community to teach the congregants novellae, especially those dealing with sins that are commonly committed by people. Rabbi Toledano writes that the more the rabbi teaches the community and causes them to repent, the more rapidly the final redemption will arrive.
To underline the importance of Torah study on Shabbat, there is a story involving two men in Marrakesh who preferred to spend their Shabbat afternoons going for leisurely strolls. After several warnings from community members, they were finally summoned to the Bet Din, which ruled that they were obligated to attend the Torah classes on Shabbat afternoon.
Summary: Napping is considered a form of Oneg Shabbat. However, one should maximize one’s time on Shabbat to study Torah.