Why people drool during sleep?

Drooling: during the day, one is able to swallow the sputum produced. At night your face muscles are relaxed and the sputum leaks or drools out of your mouth

Oromotor dysfunction: Drooling is usually caused by lack of oral control to hold oral secretions in the mouth, commonly seen in people with hypotonia, lack of muscle control. It can occur during sleep due to relaxation of oral muscles and saliva drools out of the mouth.

Saliva production: decreases during sleep. What is produced collects in your mouth because you don't swallow it like you do when you're awake. It's why you cleanse a baby's mouth & teeth after bed-time feeds, to prevent cavities from prolonged contact of sugars with teeth. Drooling is more common in mouth-breathing side-sleepers. If an adult's drooling is excessive, (s)he should see his/her primary physician.