Sleeping on the left or right side -- a scientific and halachic perspective

No worry about kochos hachitzonim here!

Summary: Considering Chazal, Rambam, and modern science. How should one conduct oneself nowadays? This is only a beginning exploration, so one should certainly NOT rely on any of this halacha leMaaseh.

Post: According to a recent short article in the New York Times, research indicates sleeping on the left side might be deleterious for the heart and heart pressure, though it might simply be people with heart problems avoided sleeping on the left side due to discomfort. Further, that sleeping on the left side significantly increases the total amount of digestive reflux time.

What does halacha have to say about sleeping on various sides? Well, there is the gemara in Berachot 13b:

R. Joseph said: A man lying on his back should not recite the Shema'. [This implies] that he may not read [the Shema' lying on his back], but there is no objection to his sleeping in this posture. But did not R. Joshua b. Levi curse anyone who slept lying on his back?²³  In reply it was said: To sleeping thus if he turns over a little on his side there is no objection, but to read the Shema' thus is forbidden even if he turns over somewhat. But R. Johanan turned over a little and read the Scripture? — R. Johanan was an exception, because he was very corpulent.

Rashi explains there:

לייט אמאן דגני אפרקיד - שמא יתקשה אברו בתוך שנתו ונראה לרבים והוא דרך גנאי:

"Lest he develop an erection within his sleep, which would be visible to the public, and this is a disgrace."

He does NOT, as far as I can make out, consider this to be because of fear of causing a seminal emission, which would be zera levatala. (Presumably because he is lying on his back, and this leads to exposure, rather than to said hardening of the ever.)

However, see Rashi to the parallel gemara, in Niddah 14a:

אפרקיד - פניו למעלה וגנות הוא שפעמים שיתקשה אבר תוך שינתו ויתגלה ועוד שידיו מונחות לו על אברו ומתחמם:

There, the additional reason is given that his hands might cause warming of the ever.

See also Tosafot there:

לייט אמאן דגני אפרקיד. פי' בקונטרס שידיו מונחות על מילתו ומתחמם ורשב"ם פירש שבגדיו נופלין על אמתו ומתחמם ומה שפי' עוד שפעמים [מתקשה ומתגלה] לא נראה דבבית סגור או אפל מאי א"ל:

The Rif cites this give and take in the gemara, but it might just be for the sake of discussing the halachos of kriyas Shema:

Rav Yosef said: One who is lying on his back should not read Shema,
This implies that he may not read, but he may, with no problem, go to sleep such!
But Rabbi Yehoshua ben Levi cursed anyone who slept lying on his back!
In terms of sleeping, if he turns a bit to the side, it is fine, but in terms of reading {Shema}, even if he turns a bit to the side it is forbidden.

But Rabbi Yochanan would turn on his side and read {Shema}?!
Rabbi Yochanan was different, because he was fat.

Looking at Ayin Mishpat, Ner Mitzvah, I see no note for it. This would be significant, because it would mean that the Tur, Shulchan Aruch, and Rema do not bring this down lehalacha. So, while it might not be a good idea, I don't know that we should necessarily resurrect this as an issur chamur.

In Gittin 47a, we read that Resh Lakish refused a mattress offered by his daughter, because "my belly is my cushion":

אמרה ליה ברתיה לא בעית מידי למזגא עליה אמר לה בתי כריסי

And see Rashi and Tosafot there. This would appear to be during eating, not during sleeping, though.

According to this website, Mishna Berura lists sleeping on back or front as an issur gadol, in siman 239 seif katan 6, though I don't see it there. This within a discussion of saying Kriyas Shma on one's back.

The Rambam does bring down halachot of sleeping on one's side, in Hilchot Deot, perek 4, halacha 5:

ז  [ה] לֹא יִישַׁן אָדָם לֹא עַל פָּנָיו, וְלֹא עַל עָרְפּוֹ, אֵלָא עַל צִדּוֹ--בִּתְחִלַּת הַלַּיְלָה עַל צַד שְׂמֹאל, וּבְסוֹף הַלַּיְלָה עַל צַד יָמִין.  וְלֹא יִישַׁן סָמוּךְ לַאֲכִילָה, אֵלָא יַמְתִּין אַחַר אֲכִילָה כְּמוֹ שָׁלוֹשׁ אוֹ אַרְבַּע שָׁעוֹת.  וְלֹא יִישַׁן בַּיּוֹם.

However, an examination of the context reveals that this is health-advice. It was not likely based on the gemara, but of positive effects of sleeping in this manner.

Aristotle spoke of benefits of sleeping on the right side:

Q. Why do men sleep better and more at ease on the right side than on the left? A. Because when they be on the left side, the lungs do lie upon and cover the heart, which is on that side under the pap; now the heart, the fountain of life, being thus occupied and hindered with the lungs, cannot exercise its own proper operation, as being overmuch heated with the lungs lying upon it, and therefore wanting the  refreshment of the air which the lungs do give it, like the blowing of a pair of  bellows, is choked and suffocated, but by lying on the right side, those inconveniences are avoided.

Compare Rambam with the recommenation of the Arab physician Avicenna:

A short sleep after a meal is useful ; one should lie first on the right side, then on the left, and finally turn back again to the right side.

though maybe this is only after a meal. I didn't take the time, yet, to fully research this. But it certainly seems, based on cultural and textual context, that Rambam is giving medical advice. I would wonder whether something similar is actually motivating Rabbi Yehoshua ben Levi.

Turning to more recent summaries of halacha, consider the following, from Halacha Yomi:

5. A person should train himself to sleep on his side. Sleeping on one's back or on one's stomach is a severe transgression. It is beneficial to begin one's sleep while lying on the left side, and afterwards to change to the right side. 

This is beneficial to one's health, since the liver is on one's right side, and the stomach on one's left. When one leans to the left, the liver will lie on the stomach and warm it with its heat. This will hasten the digestion of the food. After the food is digested, it is preferable to lean to the right, thus allowing the stomach to rest and the wastes to descend. One should not turn frequently from side to side.

Not much in the way of explanation of what the transgression is, and where it is sourced. But once we see this medieval explanation about the liver lying on the stomach and aiding digestion, and allowing the stomach to rest, etc., I wonder strongly whether this is really binding halacha. Just as we do not listen to the cures listed in the Talmud, we should not necessarily need to listen to the health-advice. And if these recommendations are based on the Rambam, and based on medieval medicine, all the more so!

And now, it seems that according to modern medicine, it is bad for the heart and for certain types of digestion to lie on one of those sides, at least for people with digestive reflux or heart conditions.

Here is another summary. Naturally enough, kabbalah comes into it. This is unfortunate because kabbalah comes and adds mystical explanations to what is often something entirely halachic or medical. And then, with the mystical rationale in place, any efforts to overturn or reevaluate the halacha are thwarted, for there is a deep mystical reason for the practice. For example, fish with meat is a halacha motivated by a medical concern, for tzaraat. Yet kabbalists added kabbalistic reasons, which would prevent a reevaluation. Even though as metzius, or evaluation of metzius, changes, the appropriate applied halacha might change as well.

The summary is from Halachically Speaking, reviewed by Rav Yisroel Belsky:

Sleeping on one's Side

In the beginning of the night, one should sleep on his left side and at the end of the night one should sleep on his right side.(67) It is much healthier to sleep on the side because doing so allows the lungs to work better, so one gets more out of his sleep.(68)

Those footnotes:

(67)Rambam Hilchos Deos 4:5, Siddur Yaavetz page 587, Kitzur Shulchan Aruch 71:5, Kaf Ha'chaim 170:72, Tov Yehoshua page 116:10, Pela Yoetz "yemin" page 283. This applies to lefties as well (refer to Yemin Moshe pages 27 and 58). The reason is because in this way his food will be digested properly (Kitzur Shulchan Aruch 71:5). (68) Horav Yisroel Belsky Shlita.

So one is the Rambam, but then Kaf Kachaim comes into it. The explanation of lungs working better may be true -- it could also prevent snoring -- but it does not mean that such working better is what was motivating the Rambam, motivating Rabbi Yehoshua ben Levi, or that such benefit of better breathing would raise this to an obligation. The intent might have been to give modern scientific rationale, but I think one should question whether it is really a halachic obligation, especially if someone finds other sorts of sleep more comfortable.

Another discussion of this, From Halachah For Today:

1)One should not sleep on his stomach nor on his back, rather on his side.

In the beginning of the night he should sleep on his left side and at the end of the night on his right side. (Rambam Hilchos Dei’os Perek 4 Halacha 5)

One of the reasons given for this is that the liver is on the right side and the stomach is on the left side, and thus when one sleeps on the left side, the liver’s heat will warm the stomach and will help the digestive system. (Sefer Aleh L’Terufah quoted in Sefer HaLikutim back of the Shabsi Frankel edition of the Rambam. See also Kitzur Shulchan Aruch Siman 71:5)

Another reason for the prohibition of sleeping on the back or on the stomach is to prevent embarrassing situations and/or nocturnal emissions while sleeping. (See Rashi to Brachos 13b Dibur Hamaschil Layit Aman. See also Pele Yoetz, “Yemin” and Shu”t Az Nidberu Vol. 6 Siman 50 regarding if these positions apply only to when going to sleep or to any time one lays down. See also Mishna Berura Siman 239:6 where he refers to sleeping on the back or stomach as an “Issur Gadol, a big sin”) 

Lastly, there are kabalistic reasons for not sleeping on the back and the stomach. (See Kaf HaChaim Siman 238:11) 

2) This Halacha applies equally to men and women (as even though the second reason cited does not apply to women, the first and third reason do. Though for women it isn’t considered a big sin if they do not follow this Halacha) 

This Halacha also applies equally to right handed people and left handed people.

Note how the kabbalistic reason, from the Kaf KaChaim, siman 238:11, would be a reason women should keep it; together with the outdated science. What is the kabbalistic reason? That it helps a lot to nullify the kochos hachitzonim, which appear to be negative forces of tumah.

At the end of the day, the halachic reasoning would still seem to apply, though I would not be persuaded by the kabbalistic or scientific reasons. But that would be the halachic reasoning for not sleeping on one's back, rather than being particular about the left side and then the right. Even within that halachic reasoning for the back and front, I am not so convinced. This deserves further analysis. Was Rabbi Yehoshua ben Levi's reason medical, or based on embarrassment / shichvas zera levatala? While Rishonim discuss this gemara, do they bring it down lehalacha? If it is based on embarrassment, perhaps the halacha is different nowadays, since we no longer sleep in the nude.

Two further interesting discussions. One going through many of the sources in a nice organized way, and another discussing if one should wake someone up (!) who is sleeping in an improper manner.