Why do I drool when I sleep?

Gravity plus an open mouth. You produce saliva all night, and when you sleep on your side or front with relaxed jaw muscles, saliva pools and escapes instead of being swallowed. MedlinePlus explains drooling generally comes down to three things: problems keeping saliva in the mouth, problems with swallowing, or making too much saliva, and sleep drooling is usually the first one, made worse by anything that forces mouth breathing, like a blocked nose, allergies, or enlarged tonsils. It is common and almost always harmless, though a chronically wet pillow can irritate the skin at the corner of your mouth. What helps: try back sleeping, treat nasal congestion before bed (saline spray, your usual allergy care), stay hydrated so saliva is not thick, and empty your nose rather than your mouth of the breathing job. Mention it to a GP if the drooling is new and heavy, one-sided, comes with facial weakness, trouble swallowing, or choking at night, or arrives alongside loud snoring with gasping pauses, since those patterns point at neurological or airway issues, and the NHS notes snoring can sometimes signal sleep apnoea, which is worth treating.