"At noon, all the participants [39 healthy, young adults] were subjected to a rigorous learning task intended to tax the hippocampus, a region of the brain that helps store fact-based memories. Both groups performed at comparable levels.
At 2 p.m., the nap group took a 90-minute siesta while the no-nap group stayed awake. Later that day, at 6 p.m., participants performed a new round of learning exercises. Those who remained awake throughout the day became worse at learning. In contrast, those who napped did markedly better and actually improved in their capacity to learn."
This study suggests that if you're trying to cram in facts you need to recall later, a same-day nap before your last-minute cram might be more helpful than powering through your day with nervous energy.
Here's yet another reason why you should consider incorporating a "siesta" into your day!