What Causes Sleeping Excessively For One With A VP Shunt History?

HI, MY NAME IS KAYLA MATHIS AND MY DAUGHTER IS 16 YEARS OLD AND SHE HAS A VP SHUNT SHE GOT IT WHEN SHE WAS THREE WEEKS OF AGES AND MYSLELF AND OTHERS THAT KNOW HER HAS A CONCERN SHE HAS BEEN SLEEPING A LOT LATELY AND AT NIGHT SHE GET A LEAST 11 HOURS A NIGHT AND SHE SLEEP ON THE SCHOOL BUS AND SHE SLEEP AT SCHOOL WE TOOK HER TO HER DOCTOR ON THE 17TH OF FEB AND THEY DONE A CAT SCAN AND THE DOCTOR SAID THAT THE SHUNT LOOKED GREAT AND IT WAS WORKING LIKE IT SHOULD BUT MY QUESTION IS WHY IS SHE SLEEPING SO MUCH?