I took mine at 8am. When I called prometric number to schedule, the operator didn't give me a choice...I heard that when you register online, you have an option. I would've taken the 12 pm time slot. There's too much traffic to get to my location at 8am; also, I'm not really an early bird...but I have to also say that my 8am exam went fine and had no real issues with 8am.
Good luck.