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Well, let's say you sleep 6 hours a night, and some one asks you: "If you slept 2 hours less each night, image what you could accomplish with that extra two hours every day. Just start sleeping 4 hours a night and eventually you'll get used to it!".

For some of us 10+ hours is a biological necessity, I've tried forcing myself to sleep exactly 8 hours a night for two weeks at a time and it destroys me mentally, every waking hour is torture because I'm so sleep deprived.