I do the same thing. I think on weekends I just want to cram as much in to the day as possible and that's why I'm up by 6:30 or 7:00. I don't purposely get up. I wake up and end up just laying there unable to go back to sleep. Plus its totally different when you know you can get up that early and have the day to yourself as opposed to having to drive to work and put up with the people/things that drive you nutts all week.