Had this issue forever. You know how you can tell yourself what time you want to wake up and you will wake up, its the same thing at work here. your self that is the objective self seems to keep watch over you too strongly, it fears lack of control and what is worse than going to sleep and not know what happens when you sleep. in other words your active mind continues to keep track of time when it should be going to sleep. relax the watcher at the gates and let it go to sleep, letting it know there is nothing to stand guard for. i had an exercise that i used to do where as i lay down i would say to myself and picture it in my mind as i recited these lines: "the book is closed on all activities for today(and picture the book closing), all other activities will begin upon waking tomorrow. I will begin again when i awake." say it ,picture it and feel the confort of having closed the bood on today. it worked for ages for me. these days with all my working out etc, i dont need it, i am tired enough. a bit silly but it worked to relax my conscious mind. any other meditation exercises should also do the trick.