I've read a few places that the key to sleeping less, and doing it without negative effects, is to make sure you are getting a longer amount of deep sleep. The best way I've been able to do this is plenty of exercise, especially weight lifting. When I'm not getting enough exercise, I can sleep for 10 hours and still not feel rested. When I do some good strength training, I go into a much deeper sleep and feel much more rested with only around 6 hours.
There's a book called the Pleasure Trap that has an excellent perspective on sleep, among many other things.