I never felt as good in early AM sessions as I did in late afternoon or evening sessions after work. I would fatigue sooner in the AM too and I felt weaker. One benefit of early AM training is if you did it fasted and were looking for maximal fat loss. This would be ideal. But I always felt like crap too.
I don't mean to say there is a "best" time as that's purely preference. You can build the same physique with AM or PM. But the science does support the idea that your body is more "primed" for weight training toward the afternoon/evening.
1. In the AM, your core temp is lower. As you know, a lower body temp means stiffer body/muscles, etc. Your hydration is also sub-optimal. To me, these two things can pose a higher risk of injury.
2. Body temp typically increases as the day progresses (as does hydration). Muscle strength and endurance are considered to peak in the late afternoon, when body temperature is highest.
3.The afternoon is also (typically) when reaction time is quickest and heart rate and blood pressure are lowest - all of this can combine to increase performance marginally. Racinais S. Scandinavian Journal of Medicine and Science in Sports, 2011, Feb.;20 Suppl 3():1600-0838.
4. Cortisol is at its peak in the morning. Cortisol + fasted state = a more catabolic environment.
5. [Applies to McNatties only] Male body produces more testosterone during late afternoon resistance training than it does during morning workouts. Hayes LD, Bickerstaff GF, Baker JS. Chronobiology International, 2010, Oct.;27(4):1525-6073.
6. Gays typically prefer Early AM workouts.