I exercise first thing in the morning because exercise is energizing. (Research also shows that moderate aerobic exercise Boosts Your Brainpower, too.)
I get up early and run. Then I cool off while I read my emails and twitter feed and am downstairs before my kids so I can eat breakfast with them. Not only will you get an energy boost, efficiency in the morning sets the stage for the rest of your day. Start your day productively and your entire day will be more productive, too.