Growing up, my parents never acknowledged Sundays as a day to relax our sacrifices during Lent. In fact, the first time I ever heard about it was during a Mass when I was almost out of high school. I do respect that Sundays are a legitimate celebration, but because of the strict practices of my youth, I would personally feel wrong about the weekly lapse. Additionally, as has been mentioned a few times already, it just wouldn't feel like a sacrifice to me if I indulged after only 6 days.
I feel like the changes I make during Lent are all things that better my mentality and health. I give up a lot of garbage that fills up daily life and alternately add healthy habits. To use Sundays as a day to embrace the "clutter" in my life would feel like a perversion of the celebration of Christ. Of course this only applies to my personal situation. For example, I have a short fuse sometimes, so that's one thing I really try to focus on during Lent. Wouldn't it be disgusting of me to use each holy Sunday as a day to allow myself to blow my top! Most people make sacrifices of certain types of food or drinks and, after all, Sunday is a feast day. So I have no ill opinions of those who do just that!
Christ be beside me in all that I do.