A constant joy in my life is watching how my kids develop and learn. They are 3y and 18 months and seem to grow in understanding and vocabulary in the time it takes me to blink. There is a great article on Cognitive Daily about new research into the part sleep plays in helping kids learn.
The current thoughts on why younger kids need more sleep is because of the shear volume of information they ingest. When they nap their brains take all this information they have gathered and put it into order, adding to and reinforcing the information stored from previous days.
Next time I feel tired at work I will take a nap and tell my boss I am just processing the extreme amounts of information I ingested earlier.