Exercising, period, is going to be a key component of a healthy lifestyle.  Often, we struggle to even fit it in our day, let alone have the opportunity to choose *when* it get's done.  However, there are some interesting studies on when the optimal time of day to exercise is, if you have that luxury.  Generally, you'll read things that say first thing in the morning is best to kickstart your day, have an elevated metabolism all day long, etc.  However, you can just as easily find information, like this study, that demonstrates small decreases in muscle hypertrophy potential when exercising first thing in the morning,

Based on the research that I've looked into, especially this study here, if you are well trained and have been exercising on a regular basis, your body will adapt to the time of day that you regularly train and your responses to training will be just as good as any other time of day you might work out.  The key here is consistency.  You can't except to see changes in your body if you aren't training regularly anyway, so the problem of time of day is rendered a "moo point" if you can get yourself into a steady routine.

Getting into a regular routine is the key to success.  Personally, I exercise in the evenings because I work with clients in the mornings, and I like to end my day on a high note.  However, in college I would work out at the crack of dawn because that was the only time that would work for me.  Do what's best for your schedule, but make it a regular thing, and make it a scheduled part of your day!  For *most* working adults, I find that the mornings before work at the best time to exercise because there are rarely interruptions, compared to evenings where meetings, happy hours, dates, movies, networking, you name it, will take up our time and our previous plans seem to slip away.

Pick a time, stick to it, and see yourself improve over time!  If you'd like to learn a little more about the importance of following routines over time, I recommend The Compound Effect as a good, short read.

-David