I noticed that I would often wake up well before my alarm if I were quite excited or anxious for something I had to wake up for. And last year, somehow, my body decided that it wanted to rise early. I spent much of the year waking up at dawn or earlier without an alarm. This changed a bit when the nights got much colder, but for the most part, I've been able to wake up within 5-10 minutes of my intended time.
I joke with people that I use my own mental anxiety as an alarm. A friend said that this is a big indicator of my intraversion-- I have no problem getting up when it is my deep personal intention to do so. I think back to when I would set three alarms because I would successively turn each of them off, or I would be late after hitting the snooze button too many times. Some of my poor past roommates remember this behavior all too well.
Now, if I have to wake up for something, I don't set an alarm. I didn't set an alarm for a dawn hike I meant to take on Friday. It seems that when I externalize my motivation/reminder to wake up, I instinctively try to defy it, even though it originated from me. I managed to get up at 4am as I intended, to make coffee, do my morning writing, dress, and have breakfast well before our meet-up time of 6:20am.
I know that it is not just a result of my sleep cycle alone-- there have been days when I've set no real intention and slept in much later than when I do have an intention.
Have you ever experimented with intention-alarms?