I was going through the exact thing with my little boy (23mnths) about a month ago. It seems it was just a stage he was going through cos it went for about 2-3 weeks and now he is back down at 8ish at the latest sometimes as early as 7ish. I don't think I changed anything although as the weather is better now in Vic we have been playing outside more in the afternoon so maybe thats wearing him out more, or maybe the fresh air helps?