Sleep apnea?
Sleep apnea, sleep apnoea or sleep apna is a sleep disorder characterized by pauses in breathing during sleep.
Further reading discovered no discussion regarding sleep apnea as a cause for over-sleeping. The article does suggest that persons with sleep apnea can fall asleep while sitting and resting or having a conversation with others. More information is available at:
http://en.wikipedia.org/wiki/Sleep_apnea
Possible, but I highly doubt it. I also sleep very well when I sleep. I rarely wake up while sleeping.