My work schedule has been very weird lately, so I put whenever I feel like it. If I have to get up really early (earlier than 4AM) then I shower the night before. If I don't need to get up so early, then I prefer to shower in the morning.
I remember a discussion between Cliff Clavin and Norm on Cheers (ancient history, I know) where they were fighting about whether it was best to shower in the morning or at night. I believe Cliff (a mailman by trade) was disgusted that people didn't shower BEFORE going to bed while Norm (I think an accountant) was disgusted that people didn't shower BEFORE going to work. Different strokes for different folks.