Help! How to study GMAT before or after a busy day..?

I feel your pain, blaze!
I work full time and am a new father, so I like to actually have time to spend with my son. I don't resort to coffee or energy drinks because the initial energy burst is followed by either nervous twitches from too much artificial energy having been introduced into my organism or an energy crash, most times its both--one followed by the other.
I typically do between 6.5 to 7 hours of sleep nightly. I could do more, but I find I'm more tired if I sleep to long and I just draaaaaag the next morning. Everyone is different in that regard. Some people are good on as little as 5, some need 9, so find that range for yourself.
Have you tried vitamins? I steer clear of the vitamins with caffeine or guarana for the same reasons mentioned above. I use a vitamin called Bio-35 by a company called Biometriks. I get it from Wal-Mart, but you may be able to get it online. It's been great for me personally, but judge for yourself. Also, try and get in at least 30 min. a day of physical exercise! That will make the biggest difference of all! DO a search on physical conditioning for the GMAT on this forum and you'll find many suggestions, evidences to support that, and testimonials of the importance of exercise for your mind and for your physical energy levels.
I like what lanter1 said about pacing yourself in increments of time. You'll be wise, however, to do practice tests and study in longer blocks of time on the weekends to get used to the real thing...it is a four-hour exam after all. You'll need the mental conditioning to survive come test day.
I would try doing 1 hour in the morning, and 1 hour at night to start.
You may be tired and it'll be tough at first, but I think you just have to decide how badly you want it. If you really want to succeed and you really want that high GMAT score, you just have to hunker down and make the sacrifice. You'll be accustomed to the new regimen in less than a week, I bet.
But just ask yourself when you feel tired...How badly do I want this?...and the motivation will flow from within.
Sounds cheesy and I sound like the zenmaster, but you're capable of achieving more than you think!
After all, we all have the exact same amount of hours during the day to use, it's how we manage them that separates the the Greats from the Goods.
Good Luck!