Why can't we get back to sleep?

Maybe I'm just paying to much attention to the statuses of my friends on Facebook or coming up with a new explanation about the brain...But I too, have experienced this random awakening in the wee early hours in the morning. Yawning and turning over that lasts until an hour or so before your scheduled alarm goes off. This annoying state of wakening described as insomnia. Either getting to sleep or staying asleep is difficult. And I wonder why? Sometimes I chalk it up to just not burning enough energy throughout the day...my body just didn't get 'x' amount of exercise, you know something like that. Or having to much on my mind that even my dreams cant suffice my soul to be content and let me get a good or decent nights rest. Because to me personally at the end of the day that's what it comes down too. That internal state of being within us. I mean we can have transmitters and receptors, chemicals that send off certain alerts and things of that sort biologically. But all in all were humans not just body's with systems and a control panel(brain) we think, make conscious decisions, whether they are good or bad and those are the qualities that make us. The thoughts and schemes and desires we constantly turn over in our heads unconsciously or not. So again, "What is it really that keeps us awake? Why can't we go back to sleep?" We have both logical explanations but what sees fit the best?
http://www.mlive.com/living/grand-rapids/index.ssf/2009/04/trouble_sleeping_experts_share.html
http://www.velveteenmind.com/velveteenmind/2008/11/whats-the-passw.html