How many hours of sleep per day do you think are needed for maximum productivity?

It's individual, but according to the stats 7 - 9 hours.

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6 hours/day but longer on weekends

I don't think there is an x amount of hours of sleep you need. It's honestly a muscle, and you can train your body to sleep for x amount of hours that you want. I used to sleep 8 hours a day, but now I'm at 4-5 hours with even more energy.