Sleep standing?

I had never heard of such a thing until last night. I always check on the kids before I go to bed, and when I opened Cassidy's door I was a bit alarmed that I didn't see her until I walked around to the side her bed and saw her feet. I thought she was awake and just being silly, but once I pulled the covers down she was sound asleep. She actually tried to get back up in bed on her own, but I guess she was too asleep to muster up enough energy to pull herself back into bed. Silly girl!