Sleepy

Can anyone explain to me how a childs sleep patterns are dictated by weekdays vs weekends? I had to get up early on Saturday, around 0630hrs. I wasn't fully out of the bedroom when there was my daughter standing in front of me. She was awake and refused to go back to bed. I had to leave her playing on her computer rather than disturb mummy. Fast forward 48 hours. I get up at 0700hrs. I move around, I bang doors, I shout. By 0715 she still hasn't stirred out of her bed. When she does come down she can hardly see her way because her eyes are still closed. I can't get a word out of her because she is so tired. How does that happen? What is it in the air of weekends that make them get up?