I go sleep around 10-11 PM, get up at 5. for the past few weeks, about twice a week, I wake up chipper as a bird at exactly 3 AM, and always after a disturbing (as yet unrecalled) dream.
I have been able to wake up at a precise time without an alarm. I found that interesting, but I did note exactly when I was falling asleep, and made a mental note: exactly 5 AM.
I am not make a mental note to wake up an 3. I find waking up at precisely at 3 AM right after a disturbing dream eerie. Any sleep experts out there have a theory?