When I nap during the day.. I have very vivid dreams... similar to those I have when I wake up in the morning and go back to sleep.

As to HOW to have a lucid during a nap... I don't know... MILD maybe?... I assume a WILD is possible too but would probably take a really long time, unless you fall asleep for a while and don't move upon awakening, ASSUMING you're body will sleep for a little more. Idk, I just confused myself.