From Valet:

You’d think that knowing how to take a nap would be easy. But it’s not. Valet has some simple advice on how to get it done and for how long.

20 minutes will increase alertness and motor skills; 40 minutes will help boot memory function.

Finding the right place and setting up your day so you can do it regularly isn’t so easy, though. Still, I’m going to try it one day.