I had that exact same thing when I was younger! I am 18 years old now and I recall so clearly how terrified I was of being the last one asleep. There was no logical reason I could figure out behind why I felt this anxiety. It was around the age of 10 for me as well and fortunatley I do not have it anymore. I used to have to watch the same movie every night to help me fall asleep, or I would watch a really boring sport on tv. Sorry not much help on what or why this happens but at least we know we aren't alone.