I shower at night. I despise blow drying my hair, so I like to let it air dry over night in the summer (in the winter I suck it up and blow dry). I've been doing this off and on since college, but when my hair is super, super short, I generally take a shower in the morning too b/c it's just easier to style.
I shower at night unless I know for certain I don't have anywhere to be right away in the mornings because I like to let my hair air dry as much as possible. Also H is a morning shower guy so there isn't time for both of us to shower on a regular work day morning unless one of us gets up before 6am. We're both better fans of the day if we don't see the day before 6am.
I shower in the morning because it helps wake me up.
I haven't always done so. When I was in college I showered at night. We had 4 girls sharing a bathroom, trying to take showers for class. It was just easier for me to take it the night before.