Bad smell of our mouth in the morning results mainly due to decreased production of saliva (dry mouth), which normally occurs during sleep. Saliva is necessary for the removal of food residues from the oral cavity. Dry mouth combined with blood resting still while sleeping (because of the supine position), leads to the spread of bacteria found in the oral cavity. That produces compounds which are responsible for the bad odor of the breath. Dry mouth may nevertheless be the symptom of another disease such as: a) renal deficiency b) autoimmune disease (e.g. Sjӧgren syndrome) and c) medication (e.g., allergy). Snoring and mouth breathing results in more dryness of the oral cavity and consequently leads to the bad smell of the mouth in the morning.
The odor may also be associated with dental problems, especially periodontal disease (gingivitis, periodontitis) due to bacteria’s accumulation.
Other diseases that may be related to bad smell are allergies (when occurring frequently), diabetes, liver problems and gastroesophageal reflux.