Not one article about power napping but several – take a look through this from Lifehacker:

http://lifehacker.com/time-your-power-nap-naturally-with-einstein-and-dalis-1476441918

That article includes the Einstein and Dali methods of the title but it also begins with links out to other rather good Lifehacker pieces about the best time for a nap – and the best duration too.