Go with your natural cycle. I am an early riser, so if I could train at 6:30 am and still be at work early enough not to fade away by the end of the day, I would. But, as it is impossible (gym doesn't open till 5:30 am, and I need to be done by like 6 am to get downtown at a reasonable time), I train right after work, starting at about 3:30-4 pm. I find it noticeably harder than the early morning training, but better than anything after that. I would just drop the barbell and curl up in a little ball under the rack if I get to the gym at 6 pm. I do a ZUmba class with my wee one at 6:10 pm once in a while, and getting going is the hardest thing I ever do in fitness, and afterwards I can't sleep so Thursday is a complete horror.