A nap is a short period of sleep, typically taken during daylight hours as an adjunct to the usual nocturnal sleep period. Naps are most often taken as a response to drowsiness during waking hours. Cultural attitudes toward napping during the work day vary; children and the elderly are often expected to nap during the day and are provided with designated periods and locations to do so, but most working adults are not and napping on the job is widely considered unacceptable. On the other hand, many cultures (especially those in hot climates) serve their largest meals at mid-day, with an official allowance for a nap period (siesta) afterwards before returning to work.
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Naps (even multiple per day) are not typically long or deep enough to provide the same restorative benefits as a continuous 6-8 hour period of sleep, and can result in the accumulation of a sleep deficit if naps are regularly used as a substitute. Naps are are also not recommended or for those suffering from insomnia or depression, as they may aggravate already disrupted rest/wake patterns.[1]
Napping has been found to be both physiologically and psychologically beneficial. Napping for 20 minutes can help refresh the mind, improve overall alertness, boost mood and increase productivity.[2] Napping may benefit the heart. In a six-year study of Greek adults, researchers found that men who took naps at least three times a week had a 37 percent lower risk of heart-related death.[3] Napping may also relieve headaches. Scientists have been investigating the benefits of napping for years: the 20-minute nap, as well as sleep durations of 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 more effective than short ones.[4] 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, Robert Stickgold, and colleagues at Harvard University for a study which showed that a midday nap reverses information overload. Reporting in Nature Neuroscience, Sara Mednick, Stickgold and colleagues also demonstrated that, in some cases, a 1-hour nap 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."[5]
A power nap, also known as a Stage 2 nap, is a short slumber of 20 minutes or less which terminates before the occurrence of deep slow-wave sleep (SWS), intended to quickly revitalize the napper. The expression "power nap" was coined by Cornell University social psychologist James Maas.[6]
The 20-minute nap increases alertness and motor skills.[7] Various durations may be recommended for power naps, which are very short compared to regular sleep. The short duration prevents nappers from sleeping so long that they enter the slow wave portion of the normal sleep cycle without being able to complete the cycle. Entering deep, slow-wave sleep and failing to complete the normal 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. In order to attain optimal post-nap performance, a Stage 2 nap must be limited to the beginning of a sleep cycle, specifically sleep stages N1 and N2, typically 18–25 minutes.
People who regularly take these short naps, or catnaps as they used to be called, may develop a good idea of the duration which works best for them, as well as which tools, environment, position, and associated factors help produce the best results. Power naps are effective even when schedules allow a full night's sleep. Mitsuo Hayashi and Tadao Hori[8] have demonstrated that a nap improves mental performance, even after a full night's sleep.
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 latter was by far the most effective in reducing driving "incidents" and subjective sleepiness. Caffeine in coffee takes up to a half-hour to have an alerting effect, hence "a short (<15min) nap will not be compromised if it is taken immediately after the coffee."[9][10][11][12]
A contemporary fad called polyphasic sleeping entails avoiding long sleeps, instead taking regularly spaced short naps. Sara Mednick, whose sleep research investigates the effects of napping, included a chapter, "Extreme Napping", in her book Take a Nap!.[13] In response to questions from readers about the "uberman" schedule of "polyphasic sleeping", she commented as follows:
This practice rests upon one important hypothesis that our biological rhythms are adaptable. This means that we can train our internal mechanisms not only when to sleep and wake, but also when to get hungry, have energy for exercise, perform mental activities. Inferred in this hypothesis is that we have the power to regulate our mood, metabolism, core body temperature, endocrine and stress response, basically everything inside this container of flesh we call home. Truly an Uberman feat![14]
Polyphasic sleep claims to be of particular benefit in cases where very long periods of wakefulness (+24 hours) are necessary and a normal circadian rhythm is impossible (such as certain cases of military duty or emergency service). Critics of polyphasic sleep dispute the notion that the human brain can simply be classically conditioned to tolerate a state of near constant wakefulness without deleterious effects. Studies by the US military showed the success rate of introducing polyphasic sleep varied widely depending on individual physiology/psychology, the amount of time provided to acclimate to the new schedule, and the type and difficulty of the tasks being performed. Attempting to acclimate to a polyphasic schedule often induced significant stresses in the participants, with a corresponding deficits in alertness and skill that became progressive and did not improve until the restoration of normal sleep. Many of the more radical cycles (such as Mednick's "Uberman Schedule", allowing only 15 minute naps every 4 hours) are so demanding that to date their only successes have been anecdotal reports from serious devotees and are widely considered to be harmful and physically unsustainable for the average person.
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