Does she live primarily inside or outside?
I read somewhere that some cats will sleep up to 17 hours a day. My roommate's ex girlfriend's cat would sleep up to 17 hours a day. My cats sleep close to 12.
I would say that if she is sleeping only/all the time when she is inside, there may be something outside that makes her feel she needs to keep her guard up.
I wouldn't worry too much, unless her eating habits change or her demeanor changes. Cats love their sleep.
I would love to be a cat!