What question is Rashi answering?
וישכב במקום ההוא, “he lay down in that place.” The Torah emphasises that he lacked the normal means to ensure a good sleep, i.e. sheets, cushion and a blanket to cover his body, even though he was very close to a town which could have furnished these items. This is why the Torah bothers to inform us that the name of that town used to be Luz when it was first founded. He was not forced to “rough” it, but seeing that he intended to get up early in the morning to continue on his journey, he chose to do this. We have proof of this when the Torah continued after relating his dream (18), with the words: וישכם יעקב בבקר, “Yaakov arose early in the morning.” If he had stayed at an inn in the town, he could not have left that early as the gates of the town were not yet open. An alternate exegesis: the reason why he stayed outside of the town overnight is that he did not know where the entrance to the town was located as we know from Judges 1,24, where it is written concerning Luz: הראנו נא מבא העיר, “please show us where the entrance to the town is located.”
How could Yaakov go to sleep in such a holy place?!
What is the physical difference between standing (or sitting) and lying down? What might that represent, spiritually?
ספר השיחות תשנ"ב ע' 141
ברוחניות הענינים: דער חלק העליון פון דעם מענטש באדייט דער רוחניות'דיקער תוכן ומהות זיינער, דער חלק התחתון שלו באדייט בעיקר זיין גשמיות.
The Lubavitcher Rebbe, Sefer HaSichos 5752 (1991) p.141
The upper part of a person's body indicates their spiritual element, while the lower part of a person's body represents primarily their physical being.
Having one's "head" on the same level as one's "body" doesn't sound like a very good thing. But since Hashem made Yaakov sleep in this place, in this way, could there be a positive way to understand it?
ספר השיחות תשנ"ב ע' 141
דוקא מצד דעם וואס דאס איז מקום המקדש, וואס דארטן איז (דערנאך געווען) תכלית ושלימות המשכת גילוי אלקות... האט דאס געבראכט "וישכב" למעליותא, די התאחדות גמורה פון מטה ומעלה דורך גילוי העצמות.
וואס דאס איז וואס דער פסוק איז גלייך ממשיך "ויחלום והנה סולם מוצב ארצה וראשו מגיע השמימה והנה מלאכי אלקים עולים ויורדים בו, והנה ה' נצב עליו וגו'", דער חיבור ואיחוד "ארצה" און "השמימה" דורך עצמו"ה...
The Lubavitcher Rebbe, Sefer HaSichos 5752 (1991) p.141
The very fact that the place where Yaakov slept was the location that the Beis HaMikdash would later be built -- and that place had the ultimate G-dly revelation -- brought to "and he lay down" in a positive way, namely, the total unity of Above and below, through the revelation of G-d's Essence.
And this is precisely what the following verse indicates: "He then dreamed that there was a ladder fixed to the ground with its top reaching up the the heavens, and there were angels of G-d ascending and descending it, and the Eternal was there, standing over him..."
The "ground" became united with the "heavens," because Hashem's Essence was revealed.