Why is you can't sleep in on the weekends?

Every weekend for the past three weeks, B wakes up some time between 6 am and 7 am. Every weekday, when he has to go to school, he sleeps until 8:30 am and then it's a mad rush for the bus. Yes, of course, I could wake him earlier and save the stress of racing out the door at the last minute but what I really want to know is WHY is it that he stays up later on Friday and Saturday nights but wakes up earlier? Those are the very mornings when I can actually sleep in and he's up and at 'em. Weekdays hold a list of chores so I get out of bed waaaaayyy earlier. B's answer? "I miss you more on weekends." Sigh.