Geez - I was just reading something about why the early AM would be superior to the PM workout. I'll try to find it.
EDIT: I can't find it, but I'll keep looking. The article was by an exercise physiologist, and though not based on specific research, was based on fairly solid projections of maximum protein systhesis and most effective fuel delivery during exercise. It was all about the impact of nutritional timing on the exercise, and not so much on environmental factors. It was a fairly slim perceived gain, though. It was a 52/48 thing, in theory. Again, from a man of science, but not based on a study.