Napping at midday, when the brain's ability to learn may have deteriorated, may clear the brain's memory "storage area" and make room for new information.
In the study, the nap group was given the chance for a 90-minute siesta at 2 p.m.; the no-nap group was asked to stay awake.
People in the group which didn't nap had a 10% reduction in their learning capacity. The people who had a nap improved their ability to learn by 10% (not much).
Image source: Sleeping kitten. Wikipedia, Tilman Piesk, public domain.