situational awareness. humans instinctually need to feel safe. sleeping with your head right by the door gives you the least amount of reaction time/space to stave off invasion. inagine a rabbit sleeping with its nose out of a rabbit hole. in modern times with modern locks and security, its not a big consideration to be faie. but the instinct is there. people like headboards on beds for the same cavelike feelibg of protection. so yea head away from the door, bed not by the door. windows used to be avoided for being drafty or leaking light.