eppy, that's really weird, I had the same thing happen to me! I would be sitting at my desk, with my arms crossed over the desk surface and my head resting on them. I would be struggling to stay awake (damn boring classes) and eventually I would enter SP; I think I even almost suffocated myself once, hehe. I didn't know it was SP at the time, though.

If only I could recreate those conditions! *goes back to school*