Can everyone go back and edit their previous comment and include how much caffeine you consume? :)
I am envious of everyone who can function normally on less than 6.5 hours/night consistently (and if you have to consume extreme amounts of caffeine to make it work, I wouldn't call that functioning normally). I can usually make it one day on 5 or 6 hours of sleep, but doing that consistently just wears me down and generally makes me feel miserable. The worst I can remember feeling in my recent life, like another poster above, is right after my second child was born, and I went a few days on effectively no sleep. And even after the initial serious struggle went away, she continued to be a handful for several months. We got some sleep at night, basically, just enough to function (with the help of a caffeine drip). I remember during those days, one random night we were lucky and actually got a full night's sleep. I was so used to being chronically sleep-deprived, it actually felt weird to be fully rested for a change.
Regular exercise generally improves my quality of sleep. However, I do pure strength training, and find that it demands me to get more sleep. In other words, I can get by on less sleep when I'm not working out.
It's interesting, as it seems like everything I've ever read about sleep, from blogs to non-fiction, starts with stats on how the overwhelming majority of people are sleep-deprived.