One frequent cause is nasal congestion. Allergies, colds, sinus infections, or a deviated septum can block nasal airflow, leading to mouth breathing at night. When the mouth stays open, saliva is more likely to escape.
This type of drooling is often accompanied by snoring, dry mouth in the morning, or restless sleep. Treating nasal irritation usually reduces the problem.
Digestive issues can also contribute. Acid reflux at night may increase saliva production as the body attempts to protect the throat. Symptoms may include heartburn, coughing, or a sour taste when lying down.