I'll add my +1 to the time of day makes a difference camp. I tend to gravitate to 50s and 60s jazz- especially the Blue Note titles that have been so beautifully repressed by Music Matters and others - in the morning with my coffee and newspaper. In the evening I need a little more juice so I switch over to modern alternative like Liz Phair, Richard Thompson and Los Lobos. Of course this is not a hard and fast rule. It's just a listening pattern I have noticed in the last year or so.