Presuming that your clock is functioning properly, it's called conditioning (think Pavlov's dogs).
For instance, after years of having to get up with my kids for their paper routes (folding papers, driving them on Sundays, etcetera), I now wake up automatically every day - even on the weekends - and it's been almost 10 years since we quit delivering papers.
Something has conditioned you to have this response. If you think about it, you may be able to identify what it is (it could be a sound, a smell or something else entirely).
If you can't pinpoint it, I wouldn't worry unless it starts disturbing you so much that it disrupts the other parts of your life. Otherwise, just relax and enjoy your ability to tune in to the time!
