I think it's just a time she would normally fall asleep when she was younger and we've stuck to that. And if she sleeps til 8am then I'm happy with that :)
She's a nightmare at the moment though, climbing up on the cabinet we have next to the sofa and TV. She's just started climbing up on the table we put in front of the glass cabinet to stop her getting in there and no amount of "NO!"s will stop her. Grr.