I've done sessions at 6am I guess it all depends in accomodating a clients needs. Is interesting thought, some areas all guys want the earliest possible, in some others late evening. So, I always leave a window of "between 8 am and midnight" but, I can go either earlier or later depending on the location and requests.

Is there a particular time of day that seems to be most popular?

I know you guys get morning wood, and I am glad you are healthy, but please, pass the coffee with milk, and the NY Times, not your friskey friend.