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:
- Does modern science agree with these facts about digestion?
- 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 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).)
- If not, then how does he expect us to lie on the right after our food digests?
- 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.