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. The expression was coined by Cornell University social psychologist James Maas.[1]
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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.
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 15–30 minutes is most effective. 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. Others may prefer to take power naps regularly even if their schedules allow a full night's sleep. Mitsuo Hayashi, PhD and Tadao Hori, PhD[1] have demonstrated that a nap improves mental performance even after a full night's sleep. New sleep sensors and sleep timers available on several mobile devices allow advocates of power naps to sleep for exactly as long as they'd like to.
For several years scientists have been investigating the benefits of napping, both the 20-minute power nap and much longer sleep durations as long as 1–2 hours. Performance across a wide range of cognitive processes has been tested. Studies demonstrate that naps are as good as a night of sleep for some types of memory tasks. A NASA study led by David F. Dinges, professor at the University of Pennsylvania School of Medicine, found that naps can improve certain memory functions and that long naps are better than short ones.[2] In that NASA study, volunteers spent several days living on one of 18 different sleep schedules, all in a laboratory setting. To measure the effectiveness of the naps, tests probing memory, alertness, response time, and other cognitive skills were used.
The National Institute of Mental Health funded a team of doctors, led by Alan Hobson, MD, Robert Stickgold, PhD, and colleagues at Harvard University for a study which showed that a midday snooze reverses information overload. Reporting in Nature Neuroscience, Sara Mednick, PhD, 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. This study also proved that, in some cases, napping could even boost performance to an individual's top levels. The NIMH team wrote "The bottom line is: we should stop feeling guilty about taking that 'power nap' at work."[3]
A caffeine nap is a short nap that is preceded by the intake of caffeine. In a driving simulator and a series of studies, Horne and Reyner investigated the effects of cold air, radio, a break with no nap, a nap, caffeine pill vs. placebo and a short nap preceded by caffeine on mildly sleep-deprived subjects. The last mentioned was by far the most effective in reducing driving "incidents" and subjective sleepiness. Caffeine in coffee takes up to half an hour to have an alerting effect, hence "a short (<15min) nap will not be compromised if it is taken immediately after the coffee."[4][5][6][7]
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