I Don't Get It

I don't get it. I went to bed, then woke up...I mean wide awake woke up. I looked at the clock-it was only 3 hours since I went to bed. Sigh. I lay there, eventually dosing, but waking every once in a while. At one point I read a chapter of my book, then tried to sleep again. I did fall asleep, but when my alarm finally went off and I was exhausted. Now, why couldn't I be wide awake when my alarm went off?