|Ignore my silly cat face.|
So, I got curious and looked up the science. Apparently, when you are sleeping lightly (that phasic and tonic REM business), you are more likely to dream. And, of course, when you nap, you aren't going to go comatose like you should when you lay down for the night. Also, you are more likely to remember the dreams during a nap because you are more likely to wake up in the middle of the REM cycle.
On another note: apparently naps will improve your ability to retain memory, but only if you dream during them. So keep dreaming, you dizzy people, you.