Power nap is a sleep which is for usually 10-30 minutes and it has to be between 1-4 pm. The reason is that if it occurs beyond 4pm, it can then trouble your sleeping at night. The purpose of power nap is to revitalize your energy so that you can do the rest of your daily activities at an active pace.
Nowadays many preschools focus at having power naps so that kids can rest for a while after the strenuous preschool activities. Many parents also make sure that their kids have a nap after coming from school. With me, it keeps on changing. In summers, I make sure my daughter sleeps because afternoons are too long. And if in summers, your kid doesn't sleep in afternoon, then she is bound to become drowsy by 6pm. But in winters, my parenting style changes. Though I know she must be taking a power nap but candidly speaking, all my efforts fail and she manages to get away. But now, after studying and hearing so much about power naps and the good effects they have on children, I am for sure going to have her take power nap.
Of course, you don't want your kid to whine in the morning that she wants to sleep more. I know how much effort does it take to ride your five year old kid on your shoulders and carry her to washroom only because she is sleep deprived and you does not want her to be late for school!
Power naps are gradually becoming a part of parenting style of every house hold. My father always focused on it while we were kids. And he clearly said that ok! one hour has passed, get up and pick up your books to read. I must say it had a clear effect on refreshing our minds after hectic school routine!