One assumes you've already considered - and discounted - stimuli such as a cuckoo clock downstairs, or the sun moving around to shine in your window at just this time?

Your question is intriguing, but I'm also worried that you may one day wish that you hadn't slept 13.5 hours a night when you were young. Life isn't long enough to read all the books you'll want to read, do all the work you'll have to do, see your children as much as you want to see them and explore the world as much as you'd like to.

Most people have a natural circadian rhythm, which dictates the best time for sleep and waking. Additionally, many people will tell you you can "train" yourself to wake up at the same time every day without an alarm clock. There's a ton of anecdotal evidence out there to suggest some people do this unknowingly (I know I have!) but I haven't seen any studies about it. So there are a few possibilities I can imagine, based on the small amount of information you've provided:

1) You have, without knowing it, "set" your internal alarm clock to wake by noon no matter what, but because a great many people in college/performing exams go through a period of time where they pull multiple all-nighters, you're still "catching up" on a disrupted sleep pattern, which is why it doesn't matter *when* you get to sleep.

2) You might have a circadian rhythm sleep disorder, such as delayed sleep phase syndrome. It doesn't sound half as likely, but it is possible.

3) There might be, as mentioned above, an environmental indicator somewhere in or around your building (it doesn't have to be your room) which wakes you up at noon. It doesn't have to be sunlight - for instance, church bells, even if they're unintrusive, can poke some people awake without their knowledge.

Best thing to do, in my opinion, is to choose when you *want* to wake, and start setting your alarm again!