Sleep seems to be very cyclical and internalized in the body, so it isn't surprising that a drastic change would have such a dramatic effect. I wonder if anyone could gain the ability to only need 6 hours of sleep by very slowly decreasing sleep over a long time (say 6-12 months). But your body will probably never adapt to this 6 hours on the weekdays, 10 hours on the weekends stuff that seems to be common.

The people I know who sleep 5-6 hours a night consistently have been doing it for a very long time and describe the feeling of sleeping 8 hours the same way I would describe sleeping 13 hours. Just feeling slow and almost like being sleep deprived all day.