What can cause a patient to get severe headaches when laying down?

Headache. When laying down, there is more blood return to the heart from the lower limbs and less blood return to the heart from the blood vessels above the heart (in the head and neck). With more blood being pumped out and more blood hanging around in the head and sinuses - the headaches could be sinus pressure or dilation of the vessel in the head. If frequency, duration, or severity increases seek md.