For optimum energivity I find an hour of sleep is best, just enough to reset your cycle. But you can't do this more than once at a time, the next day the whole 6 hours are needed.
Don't have any idea why I'm like this, but I'm told that even as a baby I would often lie in bed for hours before finally falling asleep and as a toddler I would wake up at 5am because I was put to bed so early. Nowadays a healthy 4am to 10am schedule seems best.
Oh and anyone who doesn't want to sleep as much as they should, meditation is a great way of doing it. I managed to shave 2 hours off of my daily sleep need with 10 minutes of meditation ... so essentially I averaged 4 hours a day, for something like 5 years before I got out of the meditating habit for varying reasons.