Morning in the studio has always been my favorite time, even when it's before 6 am ;). Moving, breathing, and sweating is definitely most valuable first thing in the morning. Science supports it and human nature supports it. Morning workouts set a pace for your mind and body for the entire day. Doing it first thing in the day relieves the pressure of trying to squeeze one more thing into your schedule. So your workout remains a source of stress relief rather than stress itself. As the day goes on, there seems to be more and more competition for your time and energy. I should say this: When it's all said and done, the most important thing is that you (at any point) took the time for your health and well being. And that is enormously valuable, but, if there is a way to make the question of when a little simpler, I say do it first thing... or before too many other priorities take over your day. You will reap the benefits and may find more consistency in your routine.