Why do kids wake up at 5:45am on weekends?

It's Saturday. My kids know it's Saturday. But apparently their internal clocks disagree with the concept of sleeping past 6am. Meanwhile I'm over here like "remember when we could sleep until 8?" My 9yo was literally jumping on the bed at 5:47am asking if it was time for breakfast yet. Time to invest in blackout curtains or just accept my fate?