I am one of the lucky ones who was able to score Monday off (yay!) so I woke up at around noon and decided to workout then. I think it was the combination of the lack of glycogen stored in my muscles and the fact that Insanity Max Interval Plyometrics was voted the hardest workout of the series that made me sweat puddles onto my floor. Literally. I had to mop the floor after Insanity mopped the floor with me! What I am getting at is that there are plenty of pro's and very few con's to working out in the morning before you've had anything to eat
Con's - Time
This is the killer for me. I already wake up at 5:30AM to get to work for 6:30-6:45 so waking up at 4:00AM every day just isn't realistic. Sadly, this is the reason I usually don't do it in the morning
Pro's - Increased Ketosis, Less Blood Glucose, Speeds Up Metabolism
Ton's of pro's for working out in the morning. Due to low blood glucose (as you have just fasted for - hopefully - at least 8 hours) and lower glycogen stores in your muscles your body will turn to Ketosis earlier for energy. You may find that you 'bonk' quite quickly in morning workouts, but if you push through it will be worth it! Your body will turn to fat to make energy for your muscles to use while it conserves most of your blood glucose to fuel your brain. Faster ketosis = more adipose tissue lost during a workout (and afterwards) which is what everyone wants, right? It also has the effect of jump starting your metabolism for the rest of the day which is nothing to scoff at.
Fastest way to get a 6 pack is to work out on an empty stomach!