I cannot function in everyday life with less than 6.5 hours of sleep.
I feel great with 8 hours of sleep a night. Even with 7 hours, I still feel OK. However, if I get less than 6.5 hours, I feel as if I had 3-4 drinks. I can't concentrate on anything at all.. and I will sometimes even fall over when I walk. Caffeine boosts my energy but it doesn't improve my concentration assuming I got less than 6.5 hours of sleep.
Yeah, most people probably can't function at their best with less than 6.5 hours of sleep but I can't function at all.