[paulrudy]

Studies like this one https://www.sciencenews.org/article/scientists-spinal-fluid-... suggest that visual (at least, and perhaps other) stimulation increases blood flow in the brain, and the lack thereof can increase the flow of cerebrospinal fluid, which flushes out toxic substances. This study was done with waking subjects, but the article points out that the process is much more pronounced in sleep.

Perhaps this process occurs in naps as well, helping to refresh the brain on a biochemical level.