Hi,
it is recommended that your head lies towards one of your 4 best directions (i.e. if you're standing at the end of the bed and measuring towards the headboard)- so as you said it should be along the Northwest to Southeast line, if possible. But if Northwest is one of your good directions then Northeast, Southwest and West are options as well. Just check if it's possible to place the bed in another area of your room with your head lying towards the West, for example. But I'd look first for a power position of your bed and then for the sleeping direction. If it is possible to arrange both, it's very good, indeed.
Love,
Astrid
