Don't sleep with your head in north direction. It is believed that earth's magnetism though weak might have some impact/influence in long run. When you sleep in North-South direction you are aligning your spinal chord and brain (the whole nervous system) parallel to earth's magnetic field which might have some potential influences depending on the magntidue, place, intensity, environment and person's genetic background. That is why it is always suggested to sleep with your spinal chord aligned perpendicular to earth's magnetic field so that you minimize these influences. In old purana's it is mentioned not to sleep with head in north direction for the same reason as I believe.