Time to check for pinworms. That may not be the cause but it is worth checking. To check, take a flashlight when the child is sleeping soundly and look at the rectum. Do this about three hours after the child has gone to bed. You can actually see little white worms. The life cycle of pinworms is only about two weeks but the problem is transfer of the eggs to the mouth and then the cycle starts all over again. One round of medicine, stringent cleaning of bedding, pjs and underwear for a couple of weeks and it should be a thing of the past. Went through this once with our two sons at the suggestion of a friend and the advice of our physician. Best wishes.
PS: Since I read of all the adults who grind their teeth, let me tell you what I did. During the day I found myself clinching my teeth.......... stressful job. A co-worker suggested a little self talk. I began to say to myself "Lips together, teeth apart." It worked so I tried saying the same thing to myself at night before going to sleep. It worked better for me than the mouth guard. I am sure some people are not a suggestable as others but trying it if free!.... smile