Last semester I functioned on roughly 2 hours of sleep every night (except for weekends), and I would also have to pull at least one all nighter every week. I managed to somehow function every day, but the moment the semester ended, I got sick and no longer felt like a human being, as it were. I can function pretty well at about 4 hours of sleep if I have to. If I have nothing to wake up for though, I tend to sleep about ten hours.