sleep which terminates before the occurrence of deep sleep or slow-wave sleep (SWS), intended to quickly revitalize the subject.
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.
Scientific experiments (see Benefits section below) and anecdotal evidence suggest that an average power nap duration of around 15–30 minutes is most effective[my comment: not more than 20minutes]. Any more time, and the body enters into its usual sleep cycle. People who regularly take power naps may develop a good idea of what duration works best for them, as well as what tools, environment, position, and associated factors help induce the best results.
Even better, a power nap or mid-day nap is the sunnah of the prophet that recently has been appreciated by the scientists.
Not only does a mid-day nap give relaxation to the body in day time but it is also very conducive to sleep at right.
Relaxing once during the day is very conducive to total relaxation and sleep - once you crawn into bed for the night (Kerner)
source: Wiki+ here