Hakim Chisti in "The Traditional Healers Handbook" in a section called "Gnashing of teeth while sleeping" advises, "Give a purgative for the brain and massage the neck with oil before sleeping."
In the section describing brain purgatives, he advises, "1/2 teaspoon each of chamomile and marshmallow herb. Boil in 1 1/2 cups of water for 5 minutes. Cool, strain and drink."