For a few years, I used to do a dumbbell routine in the mornings, rising at about 530 or 6am to do so. I still find the mornings the best time to do a short yoga or bodyweight/calisthenics routine.
Since I've been working out with kettlebells, I have taken to exercising after work -- usually around 6 or 630pm -- specifically, as Dan Pink mentions here, to avoid injury. A too-vigorous kettlebell routine in the morning, particularly ballistic exercises like swings, is an invitation to injury when my body is not fully warmed-up.
Also, when I did a group kettlebell class at 6:30am a few years ago, I found that I could not concentrate or focus at work for the rest of the day. I could only sit and stare at my computer screen. An afternoon/early evening workout feels good and helps me feel nicely exhausted when it's bedtime.