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.
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. Going beyond sleep stages I and II  but failing to complete a full sleep cycle can result in a phenomenon known as sleep inertia, where one feels groggy, disoriented, and even more sleepy than before beginning the nap. Brief naps (10–15 minutes) can improve alertness directly after awakening without the detrimental effects of sleep inertia associated with longer naps.
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 may prefer to take power naps regularly even if their schedules allow a full night's sleep. Mitsuo Hayashi and Tadao Hori 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 would like to.
Power naps of fewer than 30 minutes—even those as brief as 6 and 10 minutes—restore wakefulness and promote performance and learning. A 30-minute nap may also be able to reverse the hormonal impact of a night of poor sleep or reverse the damage of sleep deprivation. A University of Düsseldorf study found superior memory recall once a person had reached 6 minutes of sleep, suggesting that the onset of sleep may initiate active memory processes of consolidation which—once triggered—remains effective even if sleep is terminated.
A Flinders University study of individuals restricted to only five hours of sleep per night found a 10-minute nap was overall the most recuperative nap duration of various nap lengths they examined (lengths of 0 min, 5 min, 10 min, 20 min, and 30 minutes): the 5-minute nap produced few benefits in comparison with the no-nap control; the 10-minute nap produced immediate improvements in all outcome measures (including sleep latency, subjective sleepiness, fatigue, vigor, and cognitive performance), with some of these benefits maintained for as long as 155 minutes; the 20-minute nap was associated with improvements emerging 35 minutes after napping and lasting up to 125 minutes after napping; and the 30-minute nap produced a period of impaired alertness and performance immediately after napping, indicative of sleep inertia, followed by improvements lasting up to 155 minutes after the nap.
Naps lasting more than 30 minutes appear to be associated with a period of impaired alertness (sleep inertia) immediately after awakening that takes some time to dissipate before wakefulness and performance improve.
For several years, scientists have been investigating the benefits of napping, both the 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. 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."
Sara Mednick conducted a study experimenting on the effects of napping, caffeine, and a placebo. Her results showed that a 60-90 minute nap is more effective than caffeine in memory and cognition.
A brief period of sleep of around 15 to 20 minutes, preceded by consuming a caffeinated drink or another stimulant, may combat daytime drowsiness more effectively than napping or drinking coffee alone. A stimulant nap (or coffee nap, caffeine nap, occasionally napuccino) was discovered by British researchers, Horne and Reyner, to be more effective than regular naps in improving post-nap alertness and cognitive functioning. 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. A nap with caffeine was by far the most effective in reducing driving "incidents" and subjective sleepiness as it helps the body get rid of sleep-inducing chemical compounds known as adenosine. 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." One account suggested that it was like a "double shot of energy" from the stimulating boost from caffeine plus better alertness from napping. This procedure has been studied on sleep-deprived humans given the task of driving a motor vehicle afterwards, although it has not been studied on elderly populations.
Some companies have nap rooms to allow employees to take a power nap. This may be in a form of a nap room with a recliner, or chairs specially designed for power napping installed in a designated area. Companies with nap rooms claim that employees are happier and become more productive at work.
Similar nap rooms and stations also exist in higher education institutions. Many colleges and universities provide napping furnitures such as cots and giant bean bags in libraries for students to take a nap after a long study. At least one university has a nap room set up in a gym. Some medical schools also set up nap rooms at the teaching hospitals. The nap rooms may include sleeping pods or cots, white noise machines, and antimicrobial pillows.
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