What is classified as "sleeper" music?

So I have had this question in my mind for quite a while now "what is sleeper music?" The reason for this question is because whenever we go anywhere in the car I'm usually the one that brings the music and the fact that I'm the only one with the ipod hook-up in the car...:) anyways, Whenever I go to put on music everyone says "no don't put on heather's music, it's sleeper music and it'll put us out." So my question is: What is sleeper music? I think I listen to good music such as.... Lifehouse, Ben Lee, Jars of clay, Gavin DeGraw, Jack Johnson, John Mayer, Sheryl Crow, Josh Kelly, Jeniffer Knapp, Nickel Creek etc... I don't like screaming music or hard music or music that you can't understand. I like music that tells a story. Is that such a bad thing? So you tell me, is that music sleeper music? If it is, please tell me what constitues not sleeper music. Your help is much appreciated...And I happen to like all my music too. So there.