My dad's one of those lucky guys who did 5 hours a night his entire career (probably going back to his Navy days at least). Now that he's retired, he's a bit more leisurely and sleeps until 6:30 or so (instead of 4:50!) but often I receive emails from him at 1 or 2am, and we'll easily stay up half the night chatting when I'm visiting. He's one of the healthiest 79 year olds you'll ever meet, and not due to a particularly great diet or exercise (though both are fine in his case; he's just not a health nut). The thing that always made me jealous about him is not so much his ability to thrive on less sleep than most of us, but his ability to wake up. One beep from a digital watch alarm and he's out of bed, feet on the floor. I can't wake up to save my life. But then again, I try to get as close to 8h a night as possible, particularly as I've focused more than ever on my health and fitness.