Both of my kids do this. One has AS and one is NT and they're both under 6. We've always attributed it to growth spurts. Also, my older child sleeps, on average, about 11 hours a night, while my younger child only sleeps, on average, about 8 hours a night. They both go to bed at the same time, but the younger one just sings and plays in her bed for 2 hours sometimes, and then gets up very early.
Everyone has different sleep patterns, and sometimes they can fluctuate. I don't think it's a big deal. You can make sure your child gets enough time in bed - but you can't really make them sleep. Fresh air and exercise can help, but not always. I don't think there's really anything wrong, if that's what you're asking. You could always mention it to the doctor, but I don't think there's any real cause for concern here.