Joelyle " almost always wake up before it goes off" .. you know that happens to me most of the time !! I set the clock at 10 am and wake up at 9:45. I guess it is your biological clock that gets programmed "biologically" if you want to put it that way.
I work and study at late after noon through till night time (even passed midnight) so I guess my biological clock is way different that the normal persons one.