We've noticed our DS4 needs more sleep when he's been actually learning things...

We let ours sleep until he wants and wake up when he wants so long as he makes it to school on time. He generally sleeps about as much as we do, ~8hrs/night unless he's sick, on a growth spurt, or just starting in a new and exciting environment full of learning possibilities. One of my aunts is one of those freakish people who only needs like 5-6 hours per night, and did throughout childhood (my mom was jealous).