Why does a long nap cause such a wierd feeling afterwards?

Factual Questions
1

Ms. Plan B and I both have the same experience. If we take a short nap - less than 20 minutes - we wake up feeling refreshed and better. But if the nap lasts longer than 20 minutes it takes an hour or two to really wake up and get the cobwebs out.

Is this a common experience?

If so, what causes it?

2

there was recently a thread about this which i cant seem to find called “how can i sleep more effectively?” or something…