In most cases a dry mouth indicates that a person is breathing through their mouth while sleeping. Since a mouth-breather using only a nasal mask is not receiving adequate therapy, doctors attempt to remedy the problem by prescribing a chinstrap which helps to keep the mouth closed or a full-face mask (covering the nose and mouth) allowing a person to continue to breathe through the mouth. In some cases if you are on a particular type of medication this may cause your mouth to become extremely dry. Read the side effects to all or any medication prescribed.