I can't offer an answer, just here to say you are not alone. If I sleep on my right side (the worst side to sleep on) or flat on my back, or even flat on my left side, I will wake up coughing and have a sore throat. If I return to sleeping on my back (upright like sitting up) the soreness/coughing goes away. I suspect it is laryngopharyngeal reflux (Dr. Jamie Koufman has a good web site with free information). I find that Gaviscon Advance alginate, or sucking on the antacid tablet/cough drop, help. As someone else mentioned, I suppose sleep apnea might also be a cause, since your mouth would be open a lot. Good luck!