You feel the difference.
Sleeping 8.5 hrs (roughly)
for me is best, i feel awake all day.
Sleeping any less than that, i feel like a zombie.
you have to be wakin up early for a while to not feel like
shit later in the day...
i think everyone is different in that aspect.
Go to sleep without an alarm clock, see what time you wake up naturally...
if it's consistently the same, then thats what ur body needs.