How old are you? If you are 18-25 years old, you are still physiologically completing the transition from adolescence to adulthood, which means at some point your circadian rhythm will "shorten" and you'll start tending towards early to bed / early to rise, with both a shorter natural sleep phase and a shorter natural awake phase.

I noticed it happening to me pretty suddenly - my sleep patterns noticeably changed sometime during college despite no major lifestyle change. I wouldn't be surprised if dramatic changes can happen within a year. As far as I know it's permanent!

The nasty downside is that, whereas the adolescent body would refuse to sleep on time but once asleep continued sleeping even until noon and I always woke up refreshed, the adult body has no problem sleeping early but will wake up at a set certain time and refuse to go back to sleep, regardless of whether I'm well rested or how late I went to sleep. So on adult-mode I actually have to go to sleep on time if I want a productive day tomorrow, which was never an issue for me in adolescent-mode. "Going to bed too late" becomes equivalent to "waking up too early".

So, if you buy the circadian explanation, then don't take "waking up naturally" as a sure sign that you've slept enough!