A colorless, sticky film of bacteria plaque forms on your teeth. Dentures that aren't cleaned or don't fit properly can also harbor odor-causing bacteria and food particles. Saliva helps cleanse and moisten your mouth. A dry mouth enables dead cells to accumulate on your tongue, gums and cheeks. These cells then decompose and cause odor.
Dry mouth is natural during sleep.
But dry mouth is especially a problem if you sleep with your mouth open. Some medications and smoking can also lead to chronic dry mouth, as can a problem with your salivary glands. Chronic lung infections and lung abscesses can produce very foul-smelling breath. Several other illnesses can cause a distinctive breath odor. People with uncontrolled diabetes often have a fruity breath odor.