What Causes Severe Headaches During Sleep?

My wife suffer s with migrain headaches often they seem to always hit her in the night when she is sleeping. The left temporal vein in her head seems to always hurt before she has a migrain. She has allergies to all of the high blood pressure meds she has tried. She has had a cat scan that shows everything looking good. The Sanford doctors in sioux falls sd just keep sending her from one dr to another and we are getting nowhere other then large bills. Is there any help or suggestions??????