No duh: Naps are good for children

I have a problem with this new study by the Proceedings of the National Academy of Sciences. It states that naps help children learn. That sounds right to me - there are similar studies linking sleep to adult learning as well. 

My problem is the way the study was conducted: They studied 40 children. That doesn't sound like enough of a sample size to reach conclusive results.

Although most parents would agree that children are better learners overall after a nap, I think that if scientists want to have measurable results on this correlation, they need to find some more children.