Chronobiology is a field of biology that examines periodic (cyclic) phenomena in living organisms and their adaptation to solar- and lunar-related rhythms. 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. Other aspects include epigenetics, development, reproduction, ecology and evolution.
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:
- 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:
- Infradian rhythms, which are cycles longer than a day. Examples include circannual or annual cycles that govern migration or reproduction cycles in many plants and animals, or the human menstrual cycle.
- Ultradian rhythms, which are cycles shorter than 24 hours, such as the 90-minute REM cycle, the 4-hour nasal cycle, or the 3-hour cycle of growth hormone production.
- Tidal rhythms, commonly observed in marine life, which follow the roughly 12.4-hour transition from high to low tide and back.
- Lunar rhythms, which follow the lunar month (29.5 days). They are relevant e.g. for marine life, as the level of the tides is modulated across the lunar cycle.
- Gene oscillations – some genes are expressed more during certain hours of the day than during other hours.
Within each cycle, the time period during which the process is more active is called the acrophase. 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. 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.
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 and fruit flies.
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.
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.
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).
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.
In 2018 a study published in PLoS ONE showed how 73 psychometric indicators measured on Twitter Content follow a diurnal pattern. 
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.
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.
- Patricia J. DeCoursey; Jay C. Dunlap; Jennifer J. Loros (2003). Chronobiology. Sinauer Associates Inc. ISBN 978-0-87893-149-1.
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- Refinetti, Roberto (2006). Circadian Physiology. CRC Press/Taylor & Francis Group. ISBN 0-8493-2233-2. Lay summary
- for a description of circadian rhythms in plants by de Mairan, Linnaeus, and Darwin see 
- 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.
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- Breus, PHD, Michael (2016). The Power of When. Little Brown and Company. ISBN 978-0-316-39126-9.
- see this historical article, subscription required
- 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.
- 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.
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- 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.
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- "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."
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- U.S. Congress, Office of Technology Assessment, "Biological Rhythms: Implications for the Worker". U.S. Government Printing Office, September 1991. Washington, DC. OTA-BA-463. NTIS PB92-117589
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- "Biorhythm experiment management plan", NASA, Ames Research Center. Moffett Field, 1983.
- "Biological Rhythms and Human Adaptation to the Environment". US Army Medical Research and Materiel Command (AMRMC), US Army Research Institute of Environmental Medicine.
- Ebert, D., K.P. Ebmeier, T. Rechlin, and W.P. Kaschka, "Biological Rhythms and Behavior", Advances in Biological Psychiatry. ISSN 0378-7354
- Horne, J.A. (Jim) & Östberg, Olov (1976). A Self-Assessment Questionnaire to determine Morningness-Eveningness in Human Circadian Rhythms. International Journal of Chronobiology, 4, 97–110.
- Roenneberg, Till, Cologne (2010). Wie wir ticken – Die Bedeutung der Chronobiologie für unser Leben, Dumont, ISBN 978-3-8321-9520-5.
- The Linnean Society of London
- Halberg Chronobiology Center at the University of Minnesota, founded by Franz Halberg, the "Father of Chronobiology"
- The University of Virginia offers an online tutorial on chronobiology.
- See the Science Museum of Virginia publication Can plants tell time?
- The University of Manchester has an informative Biological Clock Web Site
- S Ertel's analysis of Chizhevsky's work