My own anecdata are somewhat conflicted on this issue. I always did well in school, understanding the key issues in a design problem (aerospace engineering) long before my classmates, and I always attributed it to my sleep habits (8-10 hrs daily).

On the other hand, some of my finest moments of clarity have come after all-night cramming sessions. It's as if you fatigue all the usual pathways and are open up to entirely new ways of seeing things.