Why do I automatically wake up without an alarm a half-hour before I am supposed to wake up, even on weekends?

May be natural. Some of us are lucky to awake before our alarm. If you are getting adequate sleep (around 8 hrs), then your brain may be telling you that you are refreshed. However, if this new, consider whether there is new stress in your life and whether you are letting it get to you to the point of depression. If so, find some help through your physician.