Occasional drooling during sleep is very common and usually completely harmless. It often happens when you sleep deeply, your facial muscles fully relax, or you’re lying on your side.

But if the drooling is new, keeps happening a lot, feels excessive, or is clearly only on one side, it’s worth paying attention. It can sometimes point to an underlying health or lifestyle issue.
Blocked or irritated nasal passages
Allergies,

colds, sinus congestion, or a deviated septum force mouth breathing at night. When the jaw relaxes completely, extra saliva just spills out. You’ll usually notice snoring, dry mouth in the morning, or restless sleep too.

Digestive problems
Nighttime acid reflux can trigger more saliva because the body tries to protect the throat from irritation.

“Nighttime acid reflux may stimulate saliva production as the body tries to neutralize irritation in the throat.”
People with reflux often feel heartburn, a sour taste in the mouth, or coughing when lying down. Eating earlier and sleeping with the head slightly raised usually helps a lot.

Oral health issues
Inflamed gums, dental problems, or throat infections make the body produce extra saliva as a defense. It’s usually temporary and improves once the irritation is treated.
In most cases drooling is no big deal. But if it starts suddenly, won’t stop, or comes with other symptoms, see a doctor or dentist. Often fixing nasal breathing, adjusting sleep position, or treating reflux solves it quickly.