12s suck, but I prefer the mornings if given the choice. I have done both. The night shift always felt like the week was one long day that never ended. When trying to sleep, I was constantly awakened by the meter reader, mailman, door to door salesmen, phone, neighborhood dogs, etc. I do the 6:00 am start now, and I appreciate the fact that I don't have to fight rush hour.
I believe that humans are diurnal animals. Do a google search for "shift work syndrome." That should give you some ammunition.