What causes foul-smelling saliva while sleeping and how can I treat it?

I have been experiencing this problem for about one year. Whenever I fall asleep—whether it is during the day or at night—a large amount of very foul-smelling saliva collects inside my mouth while I am sleeping. It is not normal saliva; it has a very unpleasant odor and makes my mouth feel extremely dirty. Sometimes the smell and the amount of saliva wake me up from sleep. When I wake up, I have to wash my mouth immediately before I can speak comfortably because the bad smell and unpleasant feeling are so severe. This problem only happens while I am asleep. When I am awake, I do not usually have bad breath or excessive saliva. I do not sleep with my mouth open, and my nose is not blocked. This condition has been affecting my comfort and sleep, and I would like to know what could be causing it and what treatment might help.