There are a few reasons for drooling in your sleep, including side sleeping and mouth breathing
during sleep. Additionally, the swallow reflex occurs much less during sleep than during waking
hours, which can lead to a buildup of saliva. That saliva can spill out of your mouth as drool
when your facial muscles relax in your sleep and your mouth falls open. Mouth breathing during
sleep may make drooling more likely, since drool can more easily escape when your mouth is open.
While you sleep, your muscles typically relax. Since the muscles around your mouth are relaxed
your mouth can be relaxed enough that saliva slips out. The position you sleep in could make you
more prone to excessive drooling. As your body produces saliva, the liquid is more likely to
escape from the front or the side of your mouth when it’s facing downward due to mere gravity.
Sleeping with your mouth open increases the amount of air that passes through your mouth. This
increased air facilitates movement and can lead to an overflow of saliva out of your mouth.
Because drool is your saliva escaping your mouth unintentionally, it's more likely to happen
when you're not consciously able to control it, like when you're sleeping. Due to this muscle
relaxation during sleep, there is no conscious effort in managing saliva and controlling the
mouth. Factors such as sleep position, tongue placement, and overall muscle tone play a role.