What the average an 21 yrs. (F) adult should sleep a night?

Question

Is 9-10 too many?

Answer 1

Recent research shows that during their 20s people's brains keep growing and become more complex. This gets reflected in increasingly mature behaviour and perceptions -- for example, feeling and showing more empathy.

So your developing cortex benefits from more sleep than you'll need once you're past your 20s. Yes, nine or ten hours a night is just fine.

Answer 2

8 should be enough