A dry mouth signifies that you have opened your mouth at night while using your PAP therapy equipment. This will let the pressure escape out of your mouth and hence prevent you from properly treating your SDB. The dryness is caused by the pressure blowing through your mouth. If you use a full face mask, your humidity setting may need to be increased a bit. If using a nasal mask, you may need to wear a chin strap or switch to a full face mask.