My Granddaughter Has Dizziness And Her Vision Keeps Going Black

My granddaughter has dizziness and her vision keeps going black and then she has to close her eyes and refocus. She also has tingling of her legs when she is sleeping. She feels the black out feeling more when she is waken up out of her sleep. It seems better if she can sleep and wake up on her own. She spends most of her mornings laying in the office at school. She has seen many doctors and no one can diagnose what it is. She is fifteen years old. This condition has been going on since May. She has had many tests and they all come back normal.