T.S.
Very normal! As kids enter puberty they go through a LOT of growing, not just physically but physiologically as well. Her body and mind are beginning the process of going from childhood to adulthood, it's a lot of work :)
I've seen the same thing with all three of my kids and I remember it myself. Especially during high school, it was SO hard getting up in the morning, and it didn't seem to matter how much sleep I got. I remember sleeping until at least noon on the weekends, and still being tired/slow when I first woke up.