A nap is a short period of sleep, typically taken during daytime hours as an adjunct to the usual nocturnal sleep period. Naps are most often taken as a response to drowsiness during waking hours. A nap is a form of biphasic or polyphasic sleep, where the latter terms also include longer periods of sleep in addition to one single period.
For years, scientists have been investigating the benefits of napping, including the 30-minute nap as well as sleep durations of 1–2 hours. Performance across a wide range of cognitive processes has been tested.
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.
It has been shown that excessive daytime sleepiness (EDS) can be improved by prescribed napping in narcolepsy. Apart from narcolepsy, it has not been demonstrated that naps are beneficial for EDS in other sleep disorders.
The state of grogginess, impaired cognition and disorientation experienced when awakening from sleep is known as sleep inertia. This state reduces the speed of cognitive tasks but has no effects on the accuracy of task performance. The effects of sleep inertia rarely last longer than 30 minutes in the absence of prior sleep deprivation.
Epidemiological research has suggested napping as a risk factor for morbidity and mortality in elderly people.
For idiopathic hypersomnia, patients typically experience sleep inertia and are unrefreshed after napping.
Main article: Power nap
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.
The 20-minute nap increases alertness and motor skills. 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 sleepier 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.
Experimental confirmation of the benefits of this brief nap comes from a Flinders University study in Australia in which 5, 10, 20, or 30-minute periods of sleep were given. The greatest immediate improvement in measures of alertness and cognitive performance came after the 10 minutes of sleep. The 20 and 30-minute periods of sleep showed evidence of sleep inertia immediately after the naps and improvements in alertness more than 30 minutes later but not to a greater level than after the 10 minutes of sleep.
People who regularly take these short naps, or catnaps, 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 have demonstrated that a nap improves mental performance, even after a full night's sleep.
A short nap preceded by the intake of caffeine was investigated by British researchers. In a driving simulator and a series of studies, Horne and Reyner looked at 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 caffeine nap 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."
A contemporary idea called polyphasic sleeping entails avoiding long periods of sleep, 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!. 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 the 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!