Among a long list of things, it could just be your natural bio-rhythm, and that's when your body feels it's time to be awake. Unfortunately, we humans haven't completely adapted to a 9-5 day. The most important factor of this is if you feel rested and ready to go. If you do, then go with it. If you don't (because you're not going to bed early enough to get about 8 hours), then look into other problems. I found out first summer of med school that my natural clock runs from 10am to 2am, because at 2am I was ready to sleep and got to sleep fairly easily, and woke up regularly at 10am, rested and feeling good. Which explains why it took me forever to get to sleep at 9pm as a kid, and sleeping til 10am was quite easy.