they say if you take a quick nap during the day you can function just fine on 6.
some people swear by polyphasic sleep, which is taking 4-6 naps during the day of 15-30 minutes at a time and that's all the sleep you get. so basically a total of 2-3 hours a day, split up throughout. that's the extreme version anyway, another protocol is 2-4 hours for your "core" sleep and then three short naps during the rest of the day.
i can't imagine adjusting to that, but if it's legit then i'd imagine you'd be pretty damn productive if you have 20 hours available during the day instead of 16. of course, i'm not sure i could find enough shit to do to fill up 20+ hours a day.