Hopefully your vet has done blood work to rule out any physical illnesses. If she is basically healthy, chances are this may be age related.
There is something in the link below about Nocturnal Restlessness. I have had many seniors in my home and when they start showing age related changes, it is usually manifested at night.
Behavior Changes & Problems in Older (Senior, Geriatric) Dogs
Another link:
http://www.medicinenet.com/pets/dog-...enior_dogs.htm