I have read many articles that list nail biting is a sign of parasites and I have found this to be true in young children.
Bed wetting, nail biting,muscles jerking, eye twitching,stomach pains, gas, bloating,etc..
I agree with the LL, some of these behaviors do sound skeptical.
And even those listed above can contribute to other causes as well but i have seen all these disappear after treatment except for body jerking while sleeping I think this is in fact due to Tapeworms or it could hold true for ascarsis.