A power nap or cat nap is a short sleep that terminates before deep sleep (slow-wave sleep; SWS). A power nap is intended to quickly revitalize the sleeper.
Cornell University social psychologist James Maas coined the term. A power nap combined with consuming caffeine is called a stimulant nap, coffee nap, caffeine nap, or nappuccino.
Characteristics
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, 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 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. Power naps are effective even when schedules allow a full night's sleep.
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 entering SWS. Depending on duration and intensity, awakenings out of SWS results in sleep inertia, a phenomenon associated with grogginess, disorientation, and even more fatigue than prior to napping. Since sleep is the most effective and beneficial recovery method from fatigue, experts recommend considering duration vs. risk of entering SWS.
Research
Potential benefits
Power naps intend to restore alertness, performance, and learning ability. A nap may also 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.
According to clinical studies among men and women, power nappers of any frequency or duration had a significantly lower mortality ratio due to heart disease than those not napping. Specifically, those occasionally napping had a 12% lower coronary mortality, whereas those systematically napping had a 37% lower coronary mortality.
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 onset 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.
The NASA Ames Fatigue Countermeasures Group studied the effects of sleep loss and jet lag, and conducts training to counter these effects. A major fatigue countermeasures recommendation consists of a 40-minute nap ("NASA nap") which empirically showed to improve flight crew performance and alertness with a 22% statistical risk of entering SWS.