Napping is much lighter sleep than nightly sleep. When napping, you don't even go through all the phases you go when going to sleep at night. I've understood that it starts straight with REM-sleep, and that's why it's also possible to WILD when having a nap.

It's also known that sensations from the outside world reflect your dream. If you hear something, you will hear that in your dream too, and usually your brain will make something up to adjust that sound in your dream.

This may be a bit confusing post, but I hope you got the ideas.