Reish Lakish lied on his stomach and asked...

Nidah 14a (Abaye): Kohanim who ride on camels may not eat Terumah. (This heats the Ever. We are concerned lest they had emissions.)

R. Yehoshua ben Levi cursed one who sleeps Perakdan (on his back; alternatively, face down).

Question: Rav Yosef taught that one may not read (before sleeping) "Shema Yisrael..." while Perakdan;

Inference: One may not read Shema while Perakdan, but he may sleep Perakdan!

Answer: One may not read Shema even when leaning slightly to the side. One may sleep that way.

Question: R. Yochanan would read Shema this way!

Answer: He was stout.

Bava Basra 74a (Rabah bar bar Chanah): I saw those who died in the Midbar sleeping Perakdan. One had his knees bent. Our guide passed under.

Pesachim 108a: Perakdan is not considered reclining.

R. Yonah (Berachos 15a DH Shnayim): Why do we permit two sleeping under one garment to turn away from each other and say Shema? One may not say Kri'as Shema lying down, even if he leans a bit to the side! One may lean while sitting, but not while lying down! We can say that this is when he lies on his back and leans a bit to the side. One may turn totally on his side. A better answer is that when it would be an exertion (to sit up), it is permitted, like for R. Yochanan, for he was fat. Here also, since he took off his clothes, it would be an exertion to get dressed again (to say Shema sitting), so he may read leaning.

Rambam (Hilchos Kri'as Shema 2:2): Everyone reads Shema the way he is, whether standing, walking, lying or riding on an animal. One may not read Shema lying face down, or on his back facing up. One may read leaning on his side. If he is very stout and he cannot turn onto his side, or he is sick, he may lean a little on his side and read.

Ra'avad: It is better that he lie. He must do so if possible.

Hagahos Maimoniyos (2): The Rambam is stringent like all the opinions (of what is Perakdan).

Kesef Mishneh and Beis Yosef (OC 63 DH u'Mah she'Chasav Chutz): The Rambam permits totally on the side, like the first opinion in R. Yonah. Rashi and the Tur agree. Slightly on the side is permitted only for stout people, for it is painful for them to turn totally on the side. Therefore, the same applies to a sick person. R. Mano'ach says that one who is not stout or sick may read only sitting, like one who stands in awe and fear. The correct text of the Ra'avad must say 'it is better that he sit.' This is not difficult for the Rambam. He does not discuss one who can sit!

Lechem Mishneh: The Rambam holds like R. Yonah's first answer. This forced him to distinguish between totally and partially on the side. The Ra'avad holds that it is better to lie face down, which shows humility, than face up, which is haughty. However, he should have explicitly said that he should lie face down! It seems that the text of the Ra'avad says 'it is better that he sit.' He holds like R. Yonah's second answer, that it is proper to read while sitting.

Tosfos ha'Rosh (Gitin 47a): Reish Lakish used to lie on his stomach, like it says in Zevachim.

Tosfos (Nidah 14a DH Afarkid): Bava Basra 74a supports that Afarkid means on the back. The Aruch says that it is face down.

Rashi (Nidah 14a DH Afarkid): One may not sleep on his back, lest he have an erection and it will be noticeable to many. Alternatively, perhaps his hands will rest on the Ever and warm it.

Tosfos (14a DH Layit): The first Perush does not explain why it is forbidden when he is alone in the house! The Rashbam says that his garments will fall on the Ever and warm it. Some say that lying on the back warms the spine and causes erections, which can lead to emissions.

Tosfos (Pesachim 108a DH Perakdan): Our Gemara shows that Perakdan means on the back. People do not eat facing down!

Shulchan Aruch (OC 63:1): One recites Shema walking, standing, lying, riding on an animal, or sitting, but not lying face down or on his back. He may read lying on his side.

Gra (472:3): The Ran is concerned for both opinions of Perakdan. Perhaps both are called Perakdan. It is clear from Nidah that we are concerned lest this warm the Ever.

Aruch ha'Shulchan (5): The Gemara and Rashi connotes that face down is worse, lest the Ever be heated and this cause an emission.

Mishnah Berurah (2): Even leaning slightly is forbidden, for he accepts the yoke of Shamayim in a haughty way.

Bi'ur Halachah (DH Aval): If he read Perakdan, b'Di'eved he was Yotzei.

Rema: This is because he is already lying, and it is an exertion to stand.

Magen Avraham (1): The exertion is to get dressed again. The Mechaber permits totally on the side. The Gemara connotes like this.

Shulchan Aruch (ibid): If he is very fat and he cannot turn on his side, or he is sick, he leans a little and reads.

Eliyahu Rabah (2): The Kesef Mishneh connotes that if a stout or sick person can sit, he must sit.

Shulchan Aruch (239:1): One reads Al Mitaso the first Parshah of Shema and blesses 'ha'Mapil Sheinah...'

Beis Yosef (DH ucheshe'Yirtzeh): R. Yonah permits saying Shma while leaning only if it is an exertion to get up. He cannot explain 'Al Mitaso' to literally mean on his bed. Rather, it is shortly before sleeping. Rashi, the Rambam and Tur permit when one is totally on his side. They can explain that he is lying on his bed.

Rema (ibid.): See above (63:1) whether one may read while lying.

Magen Avraham (5): This is for the Chiyuv Kri'as Shema. If one was Yotzei already and reads just for Shmirah, he may lie. We find that R. Zeira read repeatedly until he fell asleep. (Surely, he was lying down.)

Mishnah Berurah (6): The Rema said there that l'Chatchilah one may not lie down to read while lying, even if he turns on his side. Some are stringent to stand or sit even if they were already Yotzei Shema. If he is already lying, all permit to lean on his side and read. One should accustom himself to lie on his side. It is a great Isur to lie face down or on his back.

Chayei Adam (35:5): One should accustom himself to lie on his left side and think about Torah until he falls asleep.

Kitzur Shulchan Aruch (71:5): One should begin sleeping on his left side to hasten digestion, and end sleeping on his right, so the stomach will rest and the waste will leave. One should not turn often from side to side.

Igros Moshe (EH 1:68): Rashi explains that one may not sleep on his back, lest he have an erection and it will be disgraceful. He did not say that it will cause an emission, for this is likely only if he was thinking about women. In Nidah he gave another Perush, lest he put his hand on the Ever, which is called 'adultery of the hand.' Here also, he did not mention an emission, for it is a Sefek Sefekah.

Shevet ha'Levi (10:237 DH Kibalti): It is good to train boys to lie on the side. This conduct of Kedushah will become their nature, and they will not put their minds to it. Very small boys cannot control this.

Torah Lishmah (367): If it is difficult for one to learn from a Sefer while sitting, and wants to lie on his back or side or stomach, we find that Reish Lakish lied on his stomach. However, one should not do so too regularly, only a little until he rests.