This is involuntary action, many people drool saliva. Some people have excessive saliva production, some have scanty. It’s their natural being. Relax, it’s nothing wrong with it & it gets controlled over the period of time. Try to sleep with closed mouth or use accessories, like an hanky tied from chin to head.

Namaskaram, The body normally produces 750 ml /1 L of saliva in a day. It is generally swallowed and re-circulated in the blood stream. In deep sleep the muscles of the body tend to relax more. The muscles in the mouth tend to relax too, making it difficult to hold the saliva. It can be due to various reasons like your sleeping posture or diet . It is usually not something to worry about. A dentist might suggest an Anti-Sialagogue which you may consider. It is either that or maybe you are dreaming of something really nice. Team Shambhala.