Polyphasic sleep, a term coined by early 20th century psychologist J.S. Szymanski, refers to the practice of sleeping multiple times in a 24-hour period—usually more than two, in contrast to biphasic sleep—and does not imply any particular sleep schedule. The circadian rhythm disorder known as irregular sleep-wake pattern is an example. The term polyphasic sleep is also used by an online community which experiments with ultra-short napping to achieve more time awake each day.
An example of polyphasic sleep is found in patients with irregular sleep-wake pattern, a circadian rhythm sleep disorder which usually is caused by head injury or dementia. Much more common examples are the sleep of human infants and of many animals. Elderly humans often have disturbed sleep, including polyphasic sleep.
In their 2006 paper “The Nature of Spontaneous Sleep Across Adulthood”, Campbell and Murphy studied sleep timing and quality in young, middle-aged and older adults. They found that, in freerunning conditions, the average duration of major nighttime sleep was significantly longer in young adults than in the other groups. The paper states further:
“Whether such patterns are simply a response to the relatively static experimental conditions, or whether they more accurately reflect the natural organization of the human sleep/wake system, compared with that which is exhibited in daily life, is open to debate. However, the comparative literature strongly suggests that shorter, polyphasically-placed sleep is the rule, rather than the exception, across the entire animal kingdom (Campbell and Tobler, 1984; Tobler, 1989). There is little reason to believe that the human sleep/wake system would evolve in a fundamentally different manner. That people often do not exhibit such sleep organization in daily life merely suggests that humans have the capacity (often with the aid of stimulants such as caffeine or increased physical activity) to overcome the propensity for sleep when it is desirable, or is required, to do so.” 
In crisis and other extreme conditions, people may not be able to achieve the recommended eight hours of sleep per day. Systematic napping may be considered necessary in such situations.
Dr. Claudio Stampi, as a result of his interest in long-distance solo boat racing, has studied the systematic timing of short naps as a means of ensuring optimal performance in situations where extreme sleep deprivation is inevitable, but he does not advocate ultrashort napping as a lifestyle. Scientific American Frontiers (PBS) has reported on Stampi's 49-day experiment where a young man napped for a total of three hours per day. It purportedly shows that all stages of sleep were included. Stampi has written about his research in his book "Why We Nap: Evolution, Chronobiology, and Functions of Polyphasic and Ultrashort Sleep" (1992). In 1989 he published results of a field study in the journal Work & Stress, concluding that "polyphasic sleep strategies improve prolonged sustained performance" under continuous work situations.
The US military has studied fatigue countermeasures. An Air Force report states:
Similarly, the Canadian Marine Pilots in their trainer's handbook report that:
NASA, in cooperation with the National Space Biomedical Research Institute, has funded research on napping. Despite NASA recommendations that astronauts sleep 8 hours a day when in space, they usually have trouble sleeping 8 hours at a stretch, so the agency needs to know about the optimal length, timing and effect of naps. Professor David Dinges of the University of Pennsylvania School of Medicine led research in a laboratory setting on sleep schedules which combined various amounts of "anchor sleep," ranging from about 4 to 8 hours in length, with no nap or daily naps of up to 2.5 hours. Longer naps were found to be better, with some cognitive functions benefiting more from napping than others. Vigilance and basic alertness benefited the least while working memory benefited greatly. Naps in the individual subjects' biological daytime worked well, but naps in their nighttime were followed by much greater sleep inertia lasting up to an hour.
The Italian Air Force (Aeronautica Militare Italiana) also conducted experiments for their pilots. In schedules involving night shifts and fragmentation of duty periods through the entire day, a sort of polyphasic sleeping schedule was studied. Subjects were to perform two hours of activity followed by four hours of rest (sleep allowed), this was repeated four times throughout the 24-hour day. Although sleep was allowed, subjects adopted a schedule of not sleeping at all during the first rest period of the day. The AMI published findings that "total sleep time was substantially reduced as compared to the usual 7-8 hour monophasic nocturnal sleep" while "maintaining good levels of vigilance as shown by the virtual absence of EEG microsleeps." EEG microsleeps are measurable and usually unnoticeable bursts of sleep in the brain while a subject appears to be awake. Nocturnal sleepers who sleep poorly may be heavily bombarded with microsleeps during waking hours, limiting focus and attention.
In an early mention of systematic napping as a lifestyle, in order to gain more time awake in the day, Buckminster Fuller reportedly advocated a regimen consisting of 30 minute naps every six hours. The short article about Fuller's nap schedule in TIME in 1943 also refers to such a schedule as "intermittent sleeping", says that he maintained it for two years, and further notes:
However, it is not clear when Fuller practiced any such sleep pattern, and whether it was really as strictly periodic as claimed in that article; it has also been said that he ended this experiment because of his wife's objections.
Critics such as psychologist and software entrepreneur Piotr Wozniak consider the theory behind severe reduction of total sleep time by way of short naps unsound, claiming that there is no brain control mechanism that would make it possible to adapt to the "multiple naps" system. They say that the body will always tend to consolidate sleep into at least one solid block, and express concern that the ways in which the ultrashort nappers attempt to limit total sleep time, restrict time spent in the various stages of the sleep cycle, and disrupt their circadian rhythms, will eventually cause them to suffer the same negative effects as those with other forms of sleep deprivation and circadian rhythm sleep disorders, such as decreased mental and physical ability, increased stress and anxiety, and a weakened immune system. Wozniak further claims to have scanned the blogs of polyphasic sleepers and found that they have to choose an "engaging activity" again and again just to stay awake and that polyphasic sleep does not improve one's learning ability or creativity.
The anonymous blogger who coined the term "uberman's sleep schedule" has responded to Wozniak's criticisms of the idea of polyphasic sleep in humans. She points out, among other statements for which she finds no support, his assertion that only men do polyphasic because
PureDoxyk's response was that this was somewhat of a canard, given that, prior to reporting her experiences, she first succeeded with the Uberman schedule with the help of a female friend.