It's not the optimum, it's just about the average time an 18 year old or so needs.
Generally, anywhere between 6.30 to 8.30 is fine if you feel rested.
And you don't call that optimum ?
Nice then.
A trained person can rest for around 4 hours and be fully pep and ready, keeps em alive for as much as needed.
But that is not optimum. Average person needs between 6-8 hours to maintain optimum performance for 16 hours.
It all depends on activity and preparations for it.