once or twice in a year I sleep for more than 18 hrs!

I sometimes experience if I may call it a sleeping disorder,... symptoms don't add up to hypersomnia or chronic fatigue syndrome. I am a 31 yrs old energetic guy. I usually sleep for 6 to 8 hrs, though the timing can sometimes be erratic. Depending on when i go to sleep i wake up accordingly. ie. if its 3 in the night then i might wake up at 9 to 11. I don't usually sleep during the day. I am not usually fatigued. I even sometimes stay awake for more than 24 hrs and make up the rest with the next night. But here is the thing which amazes me the most .... once or twice in a year I sleep for more than 18 hrs... and I know that could have been more than 24 hrs if i was not disturbed by people. During that time i wake up to go to the toilet or eat and then go back to sleep. when I am done i feel very energized. My family thinks its weird ... I feel like I was in a hibernating mode. I would like to know if this is a serious medical condition to be concerned about. Ts hs ben hapn 4 7 yrs 2 m