As soon as my summer vacation started, I began sleeping in as much as possible. The first few free days I had, I would wake up at 1:00 PM, sometimes 2:00PM, after going to bed around 1 or 2:00 in the morning. Shortly after this started, I would wake up from a 12-hour sleep and find myself STILL tired. This continued, and I couldn't understand why. My parents then let me know that there actually is such thing as oversleeping, (It was news to me.)
Here's a Google-snatched article on the matter, in which you should find your particular case a little ways down.
The point is, when I felt tired after 12 hour sleeps, I realized that they couldn't be good for me. And so now I try to make sure I obtain between 8 and 10 hours a night, aiming for the prime amount of 9 hours. (Not scientifically prime, mind you, but 9 seems like a good number for me personally).
The best advice I can give you is to aim for a specific lower number via alarms. I suppose you should also use alarms even on days when you have no plans. If it doesn't work at first, I wouldn't worry, because your body just might need to adjust to this somewhat new sleep schedule. After a while, it could very well sort itself out.
Bear in mind, I'm not a doctor. I hope this helped anyway.
