I usually shower at night. I started doing this in the dorms two years ago because everyone on my floor wanted to shower in the morning, so it was easy to get in at night. And it just sort of stuck. I like showering at night now because I get all the sweat and gross feelings from the day washed off and can go to bed clean. Also, I like to work out at night, so then I just come back to my apartment, shower, and go to sleep. :)
Like kelkay, I also just shower whenever I feel like it, so if it's been a really hot day and I come home from class all sweaty, I hop in the shower and end up feeling great afterwards, no matter what time it is.
Exactly what temptationfalls said: I need my time to wake up in the morning, and the shower is the place to do it. Funny to think that I'd be immersing myself in water while I'm groggy, but I haven't drowned yet so it's all good! Plus showering in the morning helps me feel clean and fresh all day long, so it helps with confidence.
I shower at night after I've finished leaving the house for the evening. I won't make my bed dirty from my body this way. Also, I won't wake up dirty and get some extra time to sleep. If I wake up late, then I don't have to worry about appearing dirty to everyone I come in contact with because I rushed out the door.
I prefer showering at night right before I go to sleep (so sometimes I don't shower until the very middle of the night) because I don't like the feeling of going to sleep dirty as your bed is supposed to be a clean place. Also because the warm water makes me tired and I hate waking up in the morning until the very last minute.
I shower at night when I have to wash my hair because sleeping on it just makes it less frizzy and I want maximum sleep-in time. I also have random showers when needed like after the gym or before a hot date.