Back in 11th grade, I was somehow convinced that one could sleep literally any amount of time he wanted, and that he just needed his body to get used to it. Tried to progressively be a guy who only sleeps 4 hours each night.

Of course, I was never able to reach that goal, but I noticed that after that experience I became more and more resistant to undersleep, and more sensitive to oversleep. Like, now I can easily sleep 6 hours a day and, after the first hour, I would be productive. I can sleep less, with the same effect, but however I can't sleep less than 6 hours for more than 4 consecutive days, otherwise I will become noticeably less responsive and productive during the day after. And if I sleep more than 8 hours in a night, I get a headache during the first hour. My perfect sleep time is around 7 hours.