"Jacob left Beersheba and set out for Haran. When he reached a certain place, he stopped for the night because the sun had set. Taking one of the stones there, he put it under his head and lay down to sleep." (Genesis 28:10-11 NIV)
That was my first problem with this passage: who chooses to sleep with a rock as a pillow? Apparently though, resting one's head upon a hard surface was not an uncommon practice for people from many cultures (Egyptian, for one). Keeping in mind that many people slept upon the ground, I see that it is not too far a stretch that their 'pillows' might be hard as well. Anyway, that was just a side issue. The real point of this passage is Jacob's dream:
and the God of Isaac.
on which you are lying.
to the north and to the south.