That mental alarm clock, it helps and apparently you set it.
Yes, sometimes referred to as the "body clock".
I've "set mine" to around 4AM for lucid dreaming purposes. However after this, I found that I now also automatically wake up right before I set my alarm (some kind of subconscious stress about having to wake up maybe?)
For example, I seem to wake up at 6:59AM when I set my alarm to 7:00AM. I stuff you not (not always though, sometimes I wake up 10 to 15 minutes before the alarm).