Chronobiology is a field of biology that examines periodic (cyclic) phenomena in living organisms and their adaptation to solar- and lunar-related rhythms.[1] These cycles are known as biological rhythms and are also called circadian rhythms. Chronobiology comes from the ancient Greek χρόνος (chrónos, meaning "time"), and biology, which pertains to the study, or science, of life. The related terms chronomics and chronome have been used in some cases to describe either the molecular mechanisms involved in chronobiological phenomena or the more quantitative aspects of chronobiology, particularly where comparison of cycles between organisms is required.

Chronobiological studies include but are not limited to comparative anatomy, physiology, genetics, molecular biology and behavior of organisms within biological rhythms mechanics.[1] Other aspects include epigenetics, development, reproduction, ecology and evolution.

Biological clock human
Overview, including some physiological parameters, of the human circadian rhythm ("biological clock").


The variations of the timing and duration of biological activity in living organisms occur for many essential biological processes. These occur (a) in animals (eating, sleeping, mating, hibernating, migration, cellular regeneration, etc.), (b) in plants (leaf movements, photosynthetic reactions, etc.), and in microbial organisms such as fungi and protozoa. They have even been found in bacteria, especially among the cyanobacteria (aka blue-green algae, see bacterial circadian rhythms). The most important rhythm in chronobiology is the circadian rhythm, a roughly 24-hour cycle shown by physiological processes in all these organisms. The term circadian comes from the Latin circa, meaning "around" and dies, "day", meaning "approximately a day." It is regulated by circadian clocks.

The circadian rhythm can further be broken down into routine cycles during the 24-hour day:[2]

  • Diurnal, which describes organisms active during daytime
  • Nocturnal, which describes organisms active in the night
  • Crepuscular, which describes animals primarily active during the dawn and dusk hours (ex: white-tailed deer, some bats)

While circadian rhythms are defined as regulated by endogenous processes, other biological cycles may be regulated by exogenous signals. In some cases, multi-trophic systems may exhibit rhythms driven by the circadian clock of one of the members (which may also be influenced or reset by external factors). The endogenous plant cycles may regulate the activity of the bacterium by controlling availability of plant-produced photosynthate.

Many other important cycles are also studied, including:

Within each cycle, the time period during which the process is more active is called the acrophase.[3] When the process is less active, the cycle is in its bathyphase or trough phase. The particular moment of highest activity is the peak or maximum; the lowest point is the nadir. How high (or low) the process gets is measured by the amplitude.


A circadian cycle was first observed in the 18th century in the movement of plant leaves by the French scientist Jean-Jacques d'Ortous de Mairan.[4] In 1751 Swedish botanist and naturalist Carl Linnaeus (Carl von Linné) designed a flower clock using certain species of flowering plants. By arranging the selected species in a circular pattern, he designed a clock that indicated the time of day by the flowers that were open at each given hour. For example, among members of the daisy family, he used the hawk's beard plant which opened its flowers at 6:30 am and the hawkbit which did not open its flowers until 7 am.[5]

The 1960 symposium at Cold Spring Harbor Laboratory laid the groundwork for the field of chronobiology.[6]

It was also in 1960 that Patricia DeCoursey invented the phase response curve, one of the major tools used in the field since.

Franz Halberg of the University of Minnesota, who coined the word circadian, is widely considered the "father of American chronobiology." However, it was Colin Pittendrigh and not Halberg who was elected to lead the Society for Research in Biological Rhythms in the 1970s. Halberg wanted more emphasis on the human and medical issues while Pittendrigh had his background more in evolution and ecology. With Pittendrigh as leader, the Society members did basic research on all types of organisms, plants as well as animals. More recently it has been difficult to get funding for such research on any other organisms than mice, rats, humans[7][8] and fruit flies.

Recent developments

More recently, light therapy and melatonin administration have been explored by Alfred J. Lewy (OHSU), Josephine Arendt (University of Surrey, UK) and other researchers as a means to reset animal and human circadian rhythms. Additionally, the presence of low-level light at night accelerates circadian re-entrainment of hamsters of all ages by 50%; this is thought to be related to simulation of moonlight.[9]

Humans can have a propensity to be morning people or evening people; these behavioral preferences are called chronotypes for which there are various assessment questionnaires and biological marker correlations.[10]

In the second half of 20th century, substantial contributions and formalizations have been made by Europeans such as Jürgen Aschoff and Colin Pittendrigh, who pursued different but complementary views on the phenomenon of entrainment of the circadian system by light (parametric, continuous, tonic, gradual vs. nonparametric, discrete, phasic, instantaneous, respectively[11]).

There is also a food-entrainable biological clock, which is not confined to the suprachiasmatic nucleus. The location of this clock has been disputed. Working with mice, however, Fuller et al. concluded that the food-entrainable clock seems to be located in the dorsomedial hypothalamus. During restricted feeding, it takes over control of such functions as activity timing, increasing the chances of the animal successfully locating food resources.[12]

In 2018 a study published in PLoS ONE showed how 73 psychometric indicators measured on Twitter Content follow a diurnal pattern. [13]

Other fields

Chronobiology is an interdisciplinary field of investigation. It interacts with medical and other research fields such as sleep medicine, endocrinology, geriatrics, sports medicine, space medicine and photoperiodism.[14][15][16]

In spite of the similarity of the name to genuine biological rhythms, the theory and practice of biorhythms is a classic example of pseudoscience. It attempts to describe a set of cyclic variations in human behavior based on a person's birth date. It is not a part of chronobiology.[17]

See also


  1. ^ a b Patricia J. DeCoursey; Jay C. Dunlap; Jennifer J. Loros (2003). Chronobiology. Sinauer Associates Inc. ISBN 978-0-87893-149-1.
  2. ^ Nelson RJ. 2005. An Introduction to Behavioral Endocrinology. Sinauer Associates, Inc.: Massachusetts. Pg587.
  3. ^ Refinetti, Roberto (2006). Circadian Physiology. CRC Press/Taylor & Francis Group. ISBN 0-8493-2233-2. Lay summary
  4. ^ for a description of circadian rhythms in plants by de Mairan, Linnaeus, and Darwin see [1]
  5. ^
  6. ^ Leon Kreitzman; Russell G. Foster (2004). Rhythms of life: the biological clocks that control the daily lives of every living thing. New Haven, Conn: Yale University Press. ISBN 0-300-10969-5.
  7. ^ Zivkovic, Bora (2006-07-03). "ClockTutorial #2a, Forty-Five Years of Pittendrigh's Empirical Generalizations". A Blog Around the Clock. ScienceBlogs. Retrieved 2007-12-23.
  8. ^ Zivkovic, Bora (2006-05-17). "Clocks in Bacteria V". A Blog Around the Clock. ScienceBlogs. Retrieved 2007-12-23.
  9. ^ Frank, D. W.; Evans, J. A.; Gorman, M. R. (2010). "Time-Dependent Effects of Dim Light at Night on Re-Entrainment and Masking of Hamster Activity Rhythms". Journal of Biological Rhythms. 25 (2): 103–112. doi:10.1177/0748730409360890. PMID 20348461.
  10. ^ Breus, PHD, Michael (2016). The Power of When. Little Brown and Company. ISBN 978-0-316-39126-9.
  11. ^ see this historical article, subscription required
  12. ^ Fuller, Patrick M.; Jun Lu; Clifford B. Saper (2008-05-23). "Differential Rescue of Light- and Food-Entrainable Circadian Rhythms" (free abstract). Science. 320 (5879): 1074–1077. Bibcode:2008Sci...320.1074F. doi:10.1126/science.1153277. PMC 3489954. PMID 18497298. Retrieved 2008-05-30.
  13. ^ Dzogang, Fabon; Stafford Lightman; Nello Cristianini (2018-06-20). "Diurnal Variation of Psychometric Indicators in Twitter Content". PLoS ONE. 13 (6). doi:10.1371/journal.pone.0197002.
  14. ^ Postolache, Teodor T. (2005). Sports Chronobiology, An Issue of Clinics in Sports Medicine. Saunders. ISBN 978-1-4160-2769-0.
  15. ^ Ernest Lawrence Rossi, David Lloyd (1992). Ultradian Rhythms in Life Processes: Inquiry into Fundamental Principles of Chronobiology and Psychobiology. Springer-Verlag Berlin and Heidelberg GmbH & Co. K. ISBN 978-3-540-19746-1.
  16. ^ Hayes, D.K. (1990). Chronobiology: Its Role in Clinical Medicine, General Biology, and Agriculture. John Wiley & Sons. ISBN 978-0-471-56802-5.
  17. ^ "Effects of circadian rhythm phase alteration on physiological and psychological variables: Implications to pilot performance (including a partially annotated bibliography)". NASA-TM-81277. NASA. 1981-03-01. Retrieved 2011-05-25. "No evidence exists to support the concept of biorhythms; in fact, scientific data refute their existence."

Further reading

External links

Chronobiology International

Chronobiology International is a peer-reviewed scientific journal that covers all aspects of biological and medical rhythm research, chronotherapeutics, and chronoprevention of risks. It is the official journal of the International Society for Chronobiology, the American Association for Medical Chronobiology and Chronotherapeutics, and the Society for Light Treatment and Biological Rhythms. The editor-in-chief is Francesco Portaluppi (University of Ferrara). According to the Journal Citation Reports, the journal has an impact factor of 3.540 (2015) and 3.343 (2014).

Circadian rhythm

A circadian rhythm () is a natural, internal process that regulates the sleep-wake cycle and repeats roughly every 24 hours. It can refer to any biological process that displays an endogenous, entrainable oscillation of about 24 hours. These 24-hour rhythms are driven by a circadian clock, and they have been widely observed in plants, animals, fungi, and cyanobacteria.The term circadian comes from the Latin circa, meaning "around" (or "approximately"), and diēm, meaning "day". The formal study of biological temporal rhythms, such as daily, tidal, weekly, seasonal, and annual rhythms, is called chronobiology. Processes with 24-hour oscillations are more generally called diurnal rhythms; strictly speaking, they should not be called circadian rhythms unless their endogenous nature is confirmed.Although circadian rhythms are endogenous ("built-in", self-sustained), they are adjusted (entrained) to the local environment by external cues called zeitgebers (from German, "time giver"), which include light, temperature and redox cycles. In medical science, an abnormal circadian rhythm in humans is known as circadian rhythm disorder.In 2017, the Nobel Prize in Physiology or Medicine was awarded to Jeffrey C. Hall, Michael Rosbash and Michael W. Young "for their discoveries of molecular mechanisms controlling the circadian rhythm" in fruit flies.

Circadian rhythm sleep disorder

Circadian rhythm sleep disorders (CRSD) are a family of sleep disorders which affect the timing of sleep. CRSDs arise from a persistent pattern of sleep/wake disturbances that can be caused either by dysfunction in one's biological clock system, or by misalignment between one's endogenous oscillator and externally imposed cues. As a result of this mismatch, those affected by circadian rhythm sleep disorders have a tendency to fall asleep at unconventional time points in the day. These occurrences often lead to recurring instances of disturbed rest, where individuals affected by the disorder are unable to go to sleep and awaken at "normal" times for work, school, and other social obligations.

Humans, like most living organisms, have various biological rhythms. These biological clocks control processes that fluctuate daily (e.g. body temperature, alertness, hormone secretion), generating circadian rhythms. Among these physiological characteristics, our sleep-wake propensity can also be considered one of the daily rhythms regulated by the biological clock system. Our sleeping cycles are tightly regulated by a series of circadian processes working in tandem, which allow us to experience moments of consolidated sleep during the night and a long wakeful moment during the day. Conversely, disruptions to these processes and the communication pathways between them can lead to problems in sleeping patterns, which are collectively referred to as Circadian rhythm sleep disorders.

Colin Pittendrigh

Colin Pittendrigh (October 13, 1918 – March 19, 1996) was a British-born biologist who spent most of his adult life in the United States. Pittendrigh is regarded as the "father of the biological clock," and founded the modern field of chronobiology alongside Jürgen Aschoff and Erwin Bünning. He is known for his careful descriptions of the properties of the circadian clock in Drosophila and other species, and providing the first formal models of how circadian rhythms entrain (synchronize) to local light-dark cycles.He obtained his first degree of botany at University of Durham, and was assigned to wartime service as a biologist in Trinidad during World War II where he studied malaria transmission by mosquitoes. After the war, he attended Columbia University to study for his Ph.D. He later joined the faculty of Princeton University and started his chronobiology research. He also co-chaired a Mars exploration project at NASA from 1964 to 1966.The defining principle that Pittendrigh developed throughout his career was that the properties of the circadian clock are independent from those of the behaviors it controls. This gave him the freedom to study the clock through a range of physiological functions from the hatching of fruit flies to rodents' locomotor activities. He performed large series of experiments to demonstrate that circadian rhythmicity is intrinsic and independent of environmental cues. He carried out a famous and protracted debate with Frank Brown, of Northwestern University, on whether circadian timekeeping is intrinsic or environmentally driven. Pittendrigh's data and argument ultimately prevailed and sparked interest in chronobiology.Pittendrigh died from cancer on Tuesday, March 19, 1996, at his home in Bozeman, Montana. He has been regarded as one of the most influential figures in the field, and his research influences the field of chronobiology even after his death. The Society for Research on Biological Rhythms holds biennial lectures named in honor of Pittendrigh and Aschoff.

Entrainment (chronobiology)

In the study of chronobiology, entrainment occurs when rhythmic physiological or behavioral events match their period to that of an environmental oscillation. It is ultimately the interaction between circadian rhythms and the environment. A central example is the entrainment of circadian rhythms to the daily light–dark cycle, which ultimately is determined by the Earth's rotation. Exposure to certain environmental stimuli will cue a phase shift, and abrupt change in the timing of the rhythm. Entrainment helps organisms maintain an adaptive phase relationship with the environment as well as prevent drifting of a free running rhythm. This stable phase relationship achieved is thought to be the main function of entrainment.There are two general modes of entrainment: phasic and continuous. The phasic mode is when there is limited interaction with the environment to "reset" the clock every day by the amount equal to the "error", which is the difference between the environmental cycle and the organism's circadian rhythm. The continuous mode is when the circadian rhythm is continuously adjusted by the environment, usually by constant light. Two properties, the free-running period of an organism, and the phase response curve, are the main pieces of information needed to investigate individual entrainment. There are also limits to entrainment. Although there may be individual differences in this limit, most organisms have a +/- 3 hours limit of entrainment. Due to this limit, it may take several days for re-entrainment.The term entrainment is applied because the biological rhythms are endogenous: the rhythm persists even in the absence of environmental cues because it is not a learned behavior but something that is inherent in organisms. Of the several possible cues, called zeitgebers (German for 'time-givers', 'synchronizers'), which can contribute to entrainment, light has the largest impact. Units of circadian time (CT) are used to describe entrainment to refer to the relationship between the rhythm and the light signal/pulse.The activity/rest (sleep) cycle in animals is one of the circadian rhythms that normally are entrained by environmental cues. In mammals, such endogenous rhythms are generated by the suprachiasmatic nucleus (SCN) of the anterior hypothalamus. Entrainment is accomplished by altering the concentration of clock components through altered gene expression and protein stability.Circadian oscillations occur even in the cells of isolated organs such as the liver/heart as peripheral oscillators, and it is believed that they sync up with the master pacemaker in the mammalian brain, the SCN. Such hierarchical relationships are not the only ones possible: two or more oscillators may couple in order to assume the same period without either being dominant over the other(s). This situation is analogous to Huygens's pendulum clocks.

The phase of entrainment refers to the relative timing of any circadian event within the objective 24-hour day.When good sleep hygiene is insufficient, a person's lack of synchronization to night and day can have health consequences. There is some variation within normal chronotypes' entrainment; it is normal for humans to awaken anywhere from about 5 a.m. to 9 a.m. However, patients with DSPD, ASPD and non-24-hour sleep–wake disorder are improperly entrained to light/dark.

Franz Halberg

Franz Halberg (July 5, 1919 – June 9, 2013 ) was a scientist and one of the founders of modern chronobiology. He first began his experiments in the 1940s and later founded the Chronobiology Laboratories at the University of Minnesota. Halberg published many papers also in the serials of the History Commission of International Association of Geomagnetism and Aeronomy (IAGA). He also published in "Wege zur Wissenschaft, Pathways to Science". He was a member of many international bodies, he had five titles as Dr. honoris causa, and was a member of the Leibniz Sozietät der Wissenschaften zu Berlin. In the 1950s, he introduced the word circadian, which derives from the Latin about (circa) a day (diem).

Franz Halberg was nominated several times for the Nobel Prize in Physiology or Medicine. In 1988, and again in 1989, upon invitation by Professor Björn Nordenström of the Karolinska Institute in Sweden, then a member of the Nobel committee, Germaine Cornelissen, close associate of Franz, nominated Franz for the prize, highlighting the different ingredients contributed by Franz in developing the discipline of chronobiology. Professor Björn Nordenström had come to the University of Minnesota to give a major lecture and accepted Franz's invitation to come and visit his laboratory. The invitation was extended upon Björn's return to Sweden, at the Minneapolis airport where Franz and Germaine had accompanied him to continue discussions of work of mutual interest. After Nordenström left the committee, Franz's dossier assembled by Germaine was handed over to Dr. Dora K. Hayes of the U.S. Department of Agriculture, who had a colleague eligible to make nominations.


Hamsters are rodents (order Rodentia) belonging to the subfamily Cricetinae, which contains 18 species classified in seven genera. They have become established as popular small house pets. The best-known species of hamster is the golden or Syrian hamster (Mesocricetus auratus), which is the type most commonly kept as pets. Other hamster species commonly kept as pets are the three species of dwarf hamster, Campbell's dwarf hamster (Phodopus campbelli), the winter white dwarf hamster (Phodopus sungorus) and the Roborovski hamster (Phodopus roborovskii).

Hamsters are more crepuscular than nocturnal and, in the wild, remain underground during the day to avoid being caught by predators. They feed primarily on seeds, fruits, and vegetation, and will occasionally eat burrowing insects. Physically, they are stout-bodied with distinguishing features that include elongated cheek pouches extending to their shoulders, which they use to carry food back to their burrows, as well as a short tail and fur-covered feet.

Infradian rhythm

Infradian rhythm is, in chronobiology a rhythm with a duration longer than that of a circadian rhythm, i.e. with a frequency less than one cycle in 28 hours, Examples include the menstrual cycle, breeding and tidal or seasonal rhythms. Ultradian rhythms by contrast, have periods shorter than the period of a circadian rhythm. A number of infradian rhythms are known to be caused by hormone stimulation or exogenous factors for example seasonal depression, linked to the lowering of light levels during winter.

John B. Hogenesch

John B. Hogenesch (born May 29, 1967) is an American chronobiologist and Professor of Pediatrics at the Cincinnati Children's Hospital Medical Center. The primary focus of his work has been studying the network of mammalian clock genes from the genomic and computational perspective to further the understanding of circadian behavior. He is currently the Deputy Director of the Center for Chronobiology, an Ohio Eminent Scholar, and Professor of Pediatrics in the Divisions of Perinatal Biology and Immunobiology at the Cincinnati Children's Hospital Medical Center.

Jürgen Aschoff

Jürgen Walther Ludwig Aschoff (January 25, 1913 – October 12, 1998) was a German physician, biologist and behavioral physiologist. Together with Erwin Bünning and Colin Pittendrigh, he is considered to be a co-founder of the field of chronobiology.Aschoff's work in the field of chronobiology introduced the idea that shifting one's light-dark cycle can result in harmful effects, such as correlations with mental illness.

List of scientific journals

The following is a partial list of scientific journals. There are thousands of scientific journals in publication, and many more have been published at various points in the past. The list given here is far from exhaustive, only containing some of the most influential, currently publishing journals in each field. As a rule of thumb, each field should be represented by more or less than ten positions, chosen by their impact factors and other ratings.

Note: there are many science magazines that are not scientific journals, including Scientific American, New Scientist, Australasian Science and others. They are not listed here.

For periodicals in the social sciences and humanities, see list of social science journals.

M. K. Chandrashekaran

Maroli Krishnayya Chandrashekaran (4 September 1937 – 2 July 2009), also known as Shekar or MKC, was an Indian zoologist, regarded as the founder of Indian chronobiology, the study of biological rhythms of organisms. He was a fellow of the Indian Academy of Sciences, and the Third World Academy of Sciences, and in 1979 received the Shanti Swarup Bhatnagar Prize for Science and Technology. Born in Salem, Tamil Nadu, Chandrashekaran earned bachelor's and master's degrees in zoology at Presidency College, Chennai, and a PhD at the University of Madras. He served as editor of the Journal of Biosciences from 1991 to 1997 and Resonance: Journal of Science Education from 2003 to 2005.

Museum of Zoology of the University of São Paulo

The Museum of Zoology of the University of São Paulo (Portuguese: Museu de Zoologia da Universidade de São Paulo, abbreviated MZUSP) is a public natural history museum located in the historic Ipiranga district of São Paulo, Brazil. The MZUSP is an educational and research institution that is part of the University of São Paulo. The museum began at the end of the 19th century as part of the Museu Paulista; in 1941, it moved into a dedicated building. In 1969 the museum became a part of the University of São Paulo, receiving its current name.

The MZUSP has one of the largest natural-history collections in Latin America, with over 8.5 million preserved specimens of vertebrates (amphibians, mammals, birds, fish and reptiles) and invertebrates (cnidarians, insects, crustaceans, arachnids, myriapods, annelids, mollusks and other marine groups). Each collection is curated independently, and organized according to specific needs. Other facilities in the museum include a library specializing in zoology and laboratories dedicated to research in chronobiology, electron microscopy, molecular biology, histology and CT scans. MZUSP also operates the Boracéia Biological Station in the forest near Salesópolis for field research.


Nocturnality is an animal behavior characterized by being active during the night and sleeping during the day. The common adjective is "nocturnal", versus diurnal meaning the opposite.

Nocturnal creatures generally have highly developed senses of hearing, smell, and specially adapted eyesight. Such traits can help animals such as the Helicoverpa zea moths avoid predators. Some animals, such as cats and ferrets, have eyes that can adapt to both low-level and bright day levels of illumination (see metaturnal). Others, such as bushbabies and (some) bats, can function only at night. Many nocturnal creatures including tarsiers and some owls have large eyes in comparison with their body size to compensate for the lower light levels at night. More specifically, they have been found to have a larger cornea relative to their eye size than diurnal creatures to increase their visual sensitivity: in the low-light conditions. Nocturnality helps wasps, such as Apoica flavissima, avoid hunting in intense sunlight.

Diurnal animals, including squirrels and songbirds, are active during the daytime. Crepuscular species, such as rabbits, skunks, tigers, and hyenas, are often erroneously referred to as nocturnal. Cathemeral species, such as fossas and lions, are active both in the day and at night.

Phase response curve

A phase response curve (PRC) illustrates the transient change (phase response) in the cycle period of an oscillation induced by a perturbation as a function of the phase at which it is received. PRCs are used in various fields; examples of biological oscillations are the heartbeat, circadian rhythms, and the regular, repetitive firing observed in some neurons in the absence of noise.

Serge Daan

Serge Daan (11 June 1940 – 9 February 2018) was a Dutch scientist, known for his significant contributions to the field of Chronobiology.

Sleep cycle

The sleep cycle is an oscillation between the slow-wave and REM (paradoxical) phases of sleep. It is sometimes called the ultradian sleep cycle, sleep–dream cycle, or REM-NREM cycle, to distinguish it from the circadian alternation between sleep and wakefulness. In humans this cycle takes 1–2 hours.

Ultradian rhythm

In chronobiology, an ultradian rhythm is a recurrent period or cycle repeated throughout a 24-hour day. In contrast, circadian rhythms complete one cycle daily, while infradian rhythms such as the human menstrual cycle have periods longer than a day. The Oxford English Dictionary's definition of Ultradian specifies that it refers to cycles with a period shorter than a day but longer than an hour.The descriptive term ultradian is used in sleep research in reference to the 90–120 minute cycling of the sleep stages during human sleep.There is a circasemidian rhythm in body temperature and cognitive function which is technically ultradian. However, this appears to be the first harmonic of the circadian rhythm of each and not an endogenous rhythm with its own rhythm generator.

Other ultradian rhythms include blood circulation, blinking, pulse, hormonal secretions such as growth hormone, heart rate, thermoregulation, micturition, bowel activity, nostril dilation, appetite, and arousal. Ultradian rhythms of appetite require antiphasic release of Neuropeptide Y (NPY) and Corticotropin-releasing hormone (CRH), stimulating and inhibiting appetite ultradian rhythms. Recently, ultradian rhythms of arousal lasting approximately 4 hours were attributed to the dopaminergic system in mammals. When the dopaminergic system is perturbed either by use of drugs or by genetic disruption, these 4 hours rhythms can lengthen significantly into the infradian (>24hr) range, sometimes even lasting for days (>110hr) when methamphetamines are provided.Ultradian mood states in bipolar disorder cycle much faster than rapid cycling; the latter is defined as four or more mood episodes in one year, sometimes occurring within a few weeks. Ultradian mood cycling is characterized by cycles shorter than 24 hours.

Why We Nap

Why We Nap: Evolution, Chronobiology, and Functions of Polyphasic and Ultrashort Sleep (1992) is a book edited by Claudio Stampi, sole proprietor of the Chronobiology Research Institute. It is frequently mentioned by "polyphasic sleepers", as it is one of the few published books about the subject of systematic short napping in extreme situations where consolidated sleep is not possible.According to the book, in a sleep deprived condition, measurements of a polyphasic sleeper's memory retention and analytical ability show increases as compared with monophasic and biphasic sleep (but still a decrease of 12% as compared with free running sleep). According to Stampi, the improvement is due to an extraordinary evolutionary predisposition to adopt such a sleep schedule; he hypothesizes this is possibly because polyphasic sleep was the preferred schedule of ancestors of the human race for thousands of years prior to the adoption of the monophasic schedule.

According to EEG measurements collected by Dr. Stampi during a 50-day trial of polyphasic ultrashort sleep with a test subject and published in his book Why We Nap, the proportion of sleep stages remains roughly the same during both polyphasic and monophasic sleep schedules. The major differences are that the ratio of lighter sleep stages to deeper sleep stages is slightly reduced and that sleep stages are often taken out of order or not at all, that is, some naps may be composed primarily of slow wave sleep while rapid eye movement sleep dominates other naps.

ISBN 0-8176-3462-2 published by Birkhäuser, 1992, Boston

Internal rhythms
External cycles
See also
Branches of life science and biology

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