LADIES AND GENTLEMEN.
I have just engaged in an experiment, and await peer review. I anticipate they will find the same results as myself.
Hypothesis: Marth's supposed "Arched Back" in today's picture is a natural product of laying in that position.
Experiment: imitate Marth's laying position and relax body, then look down.
Results: I have found myself also naturally arching my back to have my right arm on the ground and hip turned with knees together to achieve this position.
Interpretation: There is nothing truly out of the ordinary with this stance, laying partially supine as such. In fact, it looks like he is not conscious in some way or another, as per Sakurai's sleeping comment. I do wonder, however, what WFT's REALISTIC AND SCIENTIFIC CONNECTION with Marth's literal sleeping is? His sleeping stance in the past games has been different, him leaning against his sword in the ground. This is the knocked down stance, obviously, but the word of Sakurai has said, and it is consistent with his stance and face, that he is properly understood to be sleeping in this picture. In conclusion, close enough.
Confounding variables: his breastplate armor.