Dry mouth causing cavities?

My dentist told me that if my mouth doesn't stay moist and have saliva on my teeth than I am susceptible to cavities. I have trouble sleeping with my mouth closed and have always had this problem (for 20 something years) because I have a serious deviated spetum that keeps me from breathing through my nose for long periods. I wake up with very dry dry mouth most of the time. I have recently had a lot of cavities and never have had them before. I brush my teet atleast two times a day and use mouth rinse. Is it possible that my dry mouth at night could be causing my cavities? anyone a dentist or doctor, help?