I can relate! I slept constantly for a few hours each night, but on the weekends, I slept anywhere from 8-12 hours (I would wake up, close my eyes, and BAM! It's 3 hours later.) I had a crazy headache each time, but I was what I called "pretty depressed" then. I find this to be normal though, I think.

I'm no doctor, but I think this is just you wanting a little more sleep, that's all. I'm compelled to say, though, that you should try not to make a habit of sleeping so long. For me, I overslept, had that headache, and was upset that I was sleeping my day away when I could have gotten work done. That's me though.

As for oversleeping being "dangerous," I don't know. Interesting to point though. I'd like to find out for myself!

Take care!