This ability has a formal name, "self-awakening", that you can use for a web search.  The short answer is, nobody yet fully understands how this works.

This is a common ability among people and has been the subject of scientific investigation.  Here is an abstract for a formal paper about it:

http://cat.inist.fr/?aModele=afficheN&cpsidt=2648736

This paper has been cited by 14 others more recently, which this search on the Google Scholar site summarizes:

http://scholar.google.com/scholar?hl=en&lr=&q=related:PsLaUEBjfnoJ:scholar.google.com/&um=1&ie=UTF-8&sa=X&ei=G1xETPCVO4KCsQP__OmiDA&ved=0CBUQzwIwAA

You'd have to purchase many of these articles to read them fully, but here's another research summary that's free:

http://findarticles.com/p/articles/mi_m1355/is_n3_v92/ai_19491810/

I hope this gives you a good starting point to research further!

Cheers,

Jim

I agree, but largely subject to the nature of your alarm clock. Many mechanical and electromechaniical alarm clocks make a slight sound shortly before sounding off. So do some electronic clocks. That can be a powerful clue to a sleeper near reveille. I have lived most of my life by alarm clock, and those that gave a premonitory click seldom had an opportunity to sound off properly. In fact, I got the impression that we woke more abruptly to the soft "heads up" than to those that blasted one without any warning.

This said, I think most of us have experienced the spooky awakening in anticipation, but it is unreliable. A friend of mine described awakening exactly an hour in advance. It is a difficult field to carry out controlled experiments in.

I do however suggest that you set your current clock to go off in a quiet environment where it usually stands, and listen carefully for any associated sounds. repeat the experiment several times. If you have any pets sleeping with you, watch their reactions as well. If your alarm has any kind of radio connection, such as if it is a cell phone, remember that their activity is detectable to some people and to some nearby radios.

Plainly this is a field fraught with misdirection.

Cheers,

Jon

Thank you both for your answers, very interesting.