Where the bed goes
Headboard against a solid wall, not under a window, and out of the straight line from the door. The tradition is emphatic about the last one, and the rest is the same instinct: a protected back and a clear view. What the sleep literature can speak to is quieter and more specific, and it mostly concerns temperature, noise and light.
Why a solid wall behind the head
Tradition wants support behind you. Physics adds two things: an exterior wall or a window is the coldest surface in the room and you radiate heat to it all night, and a solid wall is the better sound barrier. Both are real, and both are small compared with the room being cool and dark.
Why not under a window
That window has been playing you this whole time. Cold glass steals your body heat all night. A draft slides in through a gap you never once suspected. And the second the sun's up, it's in your face like it pays rent here, plus street noise gets first crack at you too. If the window's the only wall the bed fits against, a heavy curtain and a tall upholstered headboard cover most of it.
The door line
The tradition calls the feet-to-the-door alignment the coffin position, from the way the dead were carried out. Practically it is where hallway light and traffic land. If the layout forces it, a footboard or a low chest at the end of the bed is the traditional remedy and it also just blocks the sightline.
Which way should the head point
Here the traditions disagree with each other. Vastu prefers the head to the south or east. Feng shui schools use a personal direction from a kua number. Western folk practice says avoid facing the door. They cannot all be right, none of them measures, and choosing on the basis of the wall, the window and the door will satisfy most of them anyway.
What moves the needle
Cool, dark and quiet. Sleep research points at roughly 16 to 19 degrees Celsius, blackout at the window, and consistent timing, and those effects are far larger than any placement rule. Get those first; arrange the bed for how the room feels.
Mattress and mirrors
The tradition also avoids a mirror reflecting the bed and a beam directly overhead. Neither has evidence behind it. Both are cheap to honour if they bother you, and there is no reason to argue with a preference that costs nothing.