We learned in Brachot 13b that R' Yehoshua ben Levi "cursed one who sleeps perakdan".
There is a disagreement among the rishonim as to what perakdan means(face-up according to Rashi, Tosafot; face-down according to He'aruch, R' Chananel, and Tur; or either according to Rambam, Rashba, Meiri, and Tur. See Choshvei Shemo pg. 52).
For kriat shema, both positions are forbidden to recite it in(Shulchan Aruch OC 63:1).
Is R' Yehoshua ben Levi's(zachur letov!) statement l'halacha? And if so, according to which definition?
I think the answer lies in the reason for the "curse". I'm interested in what people's learning brings up.