A power-nap is a short sleep which terminates before the occurrence of deep sleep or slow-wave sleep (SWS), intended to quickly revitalize the subject from drowsiness. The expression power-nap was coined by Cornell University social psychologist James Maas.
Various durations are recommended for power-naps, which are very short compared to regular sleep. 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. Entering a normal sleep cycle, but failing to complete it, can result in a phenomenon known as sleep inertia, where one feels groggy, disoriented, and even more sleepy than before beginning the nap. In order to attain maximum post-nap performance, it is critical that a power-nap be limited to the beginning of a sleep cycle, specifically sleep stages I and II.
Scientific experiments (see Benefits section below) and anecdotal evidence suggest that an average power-nap duration of around 20-30 minutes is most effective. 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. Some people take power-naps out of necessity, for example, someone who doesn't get enough sleep at night and is drowsy at work may sleep during his or her lunch break. Others may prefer to take power-naps regularly even if their schedules allow a full night's sleep. Mitsuo Hayashi, Ph.D. and Tadao Hori, Ph.D. have demonstrated that a nap improves mental performance even after a full night sleep.
The National Institute of Mental Health funded a team of doctors, led by Alan Hobson, M.D., Robert Stickgold, Ph.D., and colleagues at Harvard University in a study that showed a midday snooze reverses information overload. Reporting in Nature Neuroscience, Sara Mednick, Ph.D., Stickgold and colleagues also demonstrated that "burnout" irritation, frustration and poorer performance on a mental task can set in as a day of training wears on. Their study also proved that in some cases napping could even boost performance back to morning levels. The NIMH team wrote "The bottom line is: we should stop feeling guilty about taking that "power-nap" at work.