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Kitzur Shulchan Aruch 71:5 reads, in part (translation my own):

And it's good that he'll lie at the start of his sleep on the left side and at the end on the right side, as this is good for the health of the body, for the liver rests in the right side and the stomach on the left side, so that when he leans on the left side then the liver will be over the stomach and will warm it with its warmth; thus, the food will digest quickly. After the food is digested, it's seemly for him that he'll lean on the right side, so that the stomach will rest and the food waste will descend.

As always, for practical advice, CYLOR.

A few questions:

  1. Does modern science agree with these facts about digestion?
  2. Does he mean to imply that if someone falls asleep on his left side, then his body will shift during the night and he'll wind up on his right?
    1. If so, does modern science agree? (This is independent of the above question, which is about digestion. This one has nothing to do with digestion. Rather, it's asking whether modern science grants that someone who falls asleep on his left side will wind up on his right side assuming he sleeps awhile and assuming he winds up anywhere for an extended time (i.e., does not toss and turn a lot).)
    2. If not, then how does he expect us to lie on the right after our food digests?
  3. Do other modern halachic decisors agree with this? (I don't see it in Mishna B'rura, for example, though that doesn't, of course, mean it's not there.)

Sources, where possible, please.

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maybe i will develop this into an answer, but for now, see my post from February: parsha.blogspot.com/2011/02/sleeping-on-left-or-right-side.html –  josh waxman Jun 20 '11 at 20:26
    
There are three good answers, each addressing one of my subquestions. I can accept but one, so am picking the one that answers the question I was most curious about (number 1). –  msh210 Jul 6 '11 at 16:52
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3 Answers

up vote 5 down vote accepted

Per the following health websites - sleeping on the left side avoids heartburn

http://www.ehow.com/way_5206251_sleeping-positions-better-digestion.html

http://www.livestrong.com/article/69972-sleeping-positions-better-digestion/

The Rambam in Hilchos Deios Perek 4 Halacha 5 also mentions to start off sleeping on the left and then switch over to the right.

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Thank you! (I've now edited my question to clarify that I meant other modern halachic decisors.) –  msh210 Jun 20 '11 at 18:51
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To answer your second question, when I was younger I used to deliberately go to sleep on the left side because of the RaMBa"M, and I found that I would generally roll onto my right side during the course of the night. I do still try to employ this, but I find that it doesn't always have the same result. I don't know if it has to do with my age or the fact that my sleep is regularly interrupted nowadays, VeHaMeivin YaVin, but when I was single this definitely worked, seemingly on its own.

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Your experience pretty closely matches mine, actually. That's part of why I asked 2.1. –  msh210 Jun 20 '11 at 22:24
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\2. Does he mean to imply that if someone falls asleep on his left side, then his body will shift during the night and he'll wind up on his right? ... If not, then how does he expect us to lie on the right after our food digests?

A well-respected rabbi once explained to me that this is intended to be prescriptive, not descriptive. As a result, he has trained himself to go to sleep on the left side in a pre-sleep hypnotic state, only to rotate semi-consciously to the right side before falling asleep in earnest.

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