There's a lot of natural variance in this. There's no a priori reason that your 7.5 makes more sense than somebody else's 9. Or a house cat's 12-16. It's like asking whether 5' 8" or 6' 1" is the correct height. You roll the genetic dice and they land where they land.

Some of us have always been like this. Others have been doing it for so long that we don’t even know what it feels to be fully rested anymore either. The mental fog of being continuously sleep deprived is all they remember.