Waking around the same time without an alarm Anecdotal background

I find that regardless of what time I go to bed, or how exhausted I am, I always wake up the next morning between 5 and 6am.  This is despite my alarm being consistently set for 6:15am.  I used to work in an industry, over 15 years ago, where earlier morning starts were part of the job (started work at 6am then), but none quite as drastic since then.  I do not take naps, either.

This happens irrespective of the conditions (temperature, etc).

So, I do not have a fixed time of sleep and it is not necessarily easy to get of bed at that time (some mornings here are very cold).

The question

What are the neurological triggers that cause someone to continually wake up at a set time in the morning, regardless of time of sleep and conditions?

Note - this not a self help question.  I have read the questions and answers for "What defines the easiest time to get out of bed in the morning for humans?" and "How are people able to wake themselves up after a pre-specified amount of time?" and they did not quite have the answers I am after.
Ever wondered why you wake up in the morning—even when the alarm clock isn't making jarring noises? Wonder no more. Researchers at the Salk Institute for Biological Studies have identified a new component of the biological clock, a gene responsible for starting the clock from its restful state every morning.


http://www.salk.edu/news/pressrelease_details.php?press_id=521