Hmm normally its when I'm studying... If I'm feeling sleepy I will put some more energetic stuff on, if I'm more "awake" (had enough coffee) I will put some ambient or downbeat on. It really helps me read , as long as there r no long samples In that sense no-lyric music is perfect, its "working on" totally different areas on the brain than reading (and understanding it). When there is lyric it really disturbs my ability to concentrate about what I read!!!
I also use it as background if I'm playing civilization , reading a book, cooking, cleaning (not so often I admit it) etc. but that doesn't mean I'm not really listening to it... U know what I mean??!
I usually only listen intensely (with full focus) when I just get the CD, except for when I'm in a bus or something then there is noting else to do!
and I use trance to make it on time (try at least) to my lectures in the morning... Makes me bike like a maniac!