Why do I always wake up at the same time everyday?

No matter what time I go to bed, I always, ALWAYS wake up at around six o clock. It is a horribly early hour and since more often than not I get home late and need rest, its really inconvenient. I could go to bed and at 4 and I'd still wake up two hours later, and I could never go back to sleep again, even if my body KNOWS it needs more rest. I'm not even sure if its really the sunlight or external factors that wakes me up, because its a really dark and cloudy morning today but still, like clockwork, I woke up at six. I'm getting tired of this...