Early mornings

Why do kids wake up early only on the mornings when you had a short night and really need the sleep, or when you can all sleep in.

Today 3 of the 4 kids (youngest of course) were up by 6am. I have been going to bed early all week. Like before 9 pm. In fact I have gone to bed before the kids a couple times in the past few weeks. This is not normal for me. So last night, I'm exhausted by 7:30, but I did't get home till close to 9pm and fell asleep around 10:30-11. Then Chloe was up for the day before 5(she couldn't get her zipper down on her PJ's). Ugggh, I'm sooo tired and it is only 7am. I think I miss my running around in a blur, I didn't know how tired I was. lol