They got up early when tiny, then they hit teens and wanted to stay up later and wake up later. Now it depends entirely on what they were doing the night before (the boys will sometimes be up late gaming with their friends for example) and what they have the following morning (if there is something on or if they just feel like getting up early or if they're particularly tired they will go to bed on time or at least not whinge too hard when they're dragged out).
There's still never enough hours in the day x_x