I was taught to get 7-8 hours as a child in school, but that sleep requirements decrease as you age. Once you hit physical adulthood, which can be anywhere from ~17 to ~30 (usually 18-25), it drops to 6-ish.
I can unequivocally state that I am sleep deprived on a regular basis, but I'm a college student, so it's no surprise. 4 hours a night 5 days a week is kinda hard on the ol' brain. When I'm on vacation, however, I do this novel thing called sleeping when I'm tired, and being active when I'm not. I usually end up sleeping 6 hours a night, on the dot, by just listening to what my brain is telling me.