A dry mouth, also called xerostomia, often manifests at night while asleep due to mouth breathing, according to MedicineNet. It is also a common side effect of medications that reduce saliva production. Other causes include salivary gland diseases, sleep apnea, hormonal imbalance, diabetes and autoimmune disorders.
Christian Nordqvist explains on Medical News Today that breathing through the nose does not dry the mouth, whereas breathing through the mouth causes dryness. Xerostomia occurs because of insufficient saliva and absent or reduced saliva flow, and this condition typically results from the inadequate function of the salivary glands. It is also a common side effect of certain medications and usually improves with an adjustment of dosage or a new prescription. Older people commonly experience dry mouth, as they take more medications. Xerostomia is also possibly a sign of an underlying condition, such as diabetes or Sjogren’s syndrome. Moreover, radiation therapy to the neck and head can damage the salivary glands, and chemotherapy can change the nature and amount of the saliva.
MedicineNet lists several medications that cause lower saliva production, including antidepressants, antihistamines, blood pressure medications and diuretics. A stone or infection can block a major salivary duct, which decreases salivary production, leading to dry mouth.