@sarathustrah,
Yea, I've been doing this for a long time. For some odd reason, I set my alarm clock each night but almost always get up 1 to 10 minutes before.
My understanding is that the reason for this is a combination of my Circadian Rhythm (the body's 24-hour cycle) and psychological conditioning. In my case, I live out in the sticks (no sounds) and use a digital alarm clock (no sounds from the clock). There's always the possibility that some sound is occurring of which I'm not conscious (a distant train that goes by, perhaps?) but I doubt it. I'm more inclined to believe its a result of conditioning: I'm retired military, but for years (decades?) I've been getting up at 5:00am. This morning I woke up spontaneously at 4:54.
There's lots of good information out there on the web on this, man have methods for "conditioning" your body - wean you off the alarm clock.