The power nap is thought to maximize the benefits of sleep versus time. It is used to supplement normal sleep, especially when a sleeper has accumulated a sleep deficit.
The short duration of a power nap is designed to prevent nappers from sleeping so long that they enter a normal sleep cycle without being able to complete it. Power naps restore wakefulness and promote performance and learning.

see link : "http://en.wikipedia.org/wiki/Power_nap"

Einstein and Dali's Key Method :
http://lifehacker.com/time-your-power-nap-naturally-with-einstein-and-dalis-1476441918
https://www.fastcompany.com/3023078/leadership-now/how-dali-einstein-and-aristotle-perfected-the-power-nap

Power Nap is easy to use.
1- Setup maximal nap duration.
2- Put the device in your hand and start to sleep.
3- If the device fall down, you are in the deep sleep.
4- It's time to wake up.

Follow us https://twitter.com/app_PowerNap
http://power-nap.jimdo.com/

just for fun :
Power Nap - Music Video https://www.youtube.com/watch?v=KgphH2Y2XFo

picture by Bryon Lippincott