Why am I sleeping SO much now?

Lately I've been goin to sleep at around 12 am and gettin up at 5:35 a.m and felling absolutely FINE. But now for the past week I've been sleepin till 11 am or later. What I want to know is WHY I could be sleeping in so much, and how can I get back in the habit of gettin up early agian? I'm 19 year old guy by the way.