My 6 year old still doesn't sleep that well. In fact, he was awake for over an hour in the middle of the night last night. He says his mind won't go back to sleep, so his body can't either.
As a baby, he was colicky and NEVER slept. I've often wondered if it had anything to do with his overactive brain! When he would finally sleep, he had to be right next to me. We did the co-sleeping thing as well...out of necessity. It was the only way to get some sleep. Great bonding time too, but we were all very tired. To get him to sleep, we used to have to turn music up quite loudly (not lullaby music, loud guitar solos) and dance with him. Eventually he'd fall asleep, but usually not for long.
On a positive note, he does sleep in his own bed now and he only wakes up a couple of nights/week. Sometimes it's for a half hour, sometimes it's a couple of hours. Tough days when he has to go to school!
From what I understand, it's not unusual for gifted children to have sleep issues.