The Gemara Sanhedrin 71a quotes Rabi Zeira who says that if one sleeps in the Beis Medrash, he will forget his learning and only remember it in gaps.
We know of countless Gedolim, who, because of their total devotion to Torah, often slept in the Beis Medrash. How are we to understand this?
Shea Schwartz
In Shulchan Aruch (Yoreh De'ah 246:15), where this Halachah is quoted, the Shach asks that from Orach Chaim (151:3) it seems that only a Shinas Keva in the Beis ha'Midrash is prohibited. He therefore suggests that the Isur to doze off is only a Midas Chasidus. In addition, the Levush (Orach Chaim ibid.) writes that for a Talmid Chacham even a Shinas Kevah is Mutar (as we see learn Megilah 28b, "Beisa d'Rabanan").
It is possible that the Gemara here is limited to one who sleeps in the middle of learning. This would explain the punishment of Kera'im Kera'im as well.
D. Zupnik