A circadian rhythm, or circadian cycle, is a natural oscillation that repeats roughly every 24 hours. Circadian rhythms can refer to any process that originates within an organism and responds to the environment. Circadian rhythms are regulated by a circadian clock whose primary function is to rhythmically co-ordinate biological processes so they occur at the correct time to maximise the fitness of an individual. Circadian rhythms have been widely observed in animals, plants, fungi and cyanobacteria and there is evidence that they evolved independently in each of these kingdoms of life.
Chronobiology is a field of biology that examines timing processes, including periodic (cyclic) phenomena in living organisms, such as their adaptation to solar- and lunar-related rhythms. These cycles are known as biological rhythms. Chronobiology comes from the ancient Greek χρόνος, 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.
The suprachiasmatic nucleus or nuclei (SCN) is a small region of the brain in the hypothalamus, situated directly above the optic chiasm. It is the principal circadian pacemaker in mammals, responsible for generating circadian rhythms. Reception of light inputs from photosensitive retinal ganglion cells allow it to coordinate the subordinate cellular clocks of the body and entrain to the environment. The neuronal and hormonal activities it generates regulate many different body functions in an approximately 24-hour cycle.
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.
Jürgen Walther Ludwig Aschoff 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.
A chronobiotic is an agent that can cause phase adjustment of the circadian rhythm. That is, it is a substance capable of therapeutically entraining or re-entraining long-term desynchronized or short-term dissociated circadian rhythms in mammals, or prophylactically preventing their disruption following an environmental insult such as is caused by rapid travel across several time zones. The most widely recognized chronobiotic is the hormone melatonin, secreted at night in both diurnal and nocturnal species.
Michael Menaker, was an American chronobiology researcher, and was Commonwealth Professor of Biology at University of Virginia. His research focused on circadian rhythmicity of vertebrates, including contributing to an understanding of light input pathways on extra-retinal photoreceptors of non-mammalian vertebrates, discovering a mammalian mutation for circadian rhythmicity, and locating a circadian oscillator in the pineal gland of bird. He wrote almost 200 scientific publications.
Patricia Jackson DeCoursey was an American biologist. A leading researcher in the field of chronobiology, her research focused on behavioral, physiological, and ecological aspects of mammalian circadian rhythms. She is credited with creating the first Phase Response Curve (PRC). PRC’s are used throughout the field today to help illustrate the change of a biological oscillation in response to an external stimulus. She worked as a biology professor at the University of South Carolina (USC) from 1967 until her retirement as director of the W. Gordon Belser Arboretum in 2019.
Ueli Schibler is a Swiss biologist, chronobiologist and a professor at the University of Geneva. His research has contributed significantly to the field of chronobiology and the understanding of circadian clocks in the body. Several of his studies have demonstrated strong evidence for the existence of robust, self-sustaining circadian clocks in the peripheral tissues.
Hitoshi Okamura is a Japanese scientist who specializes in chronobiology. He is currently a professor of Systems Biology at Kyoto University Graduate School of Pharmaceutical Sciences and the Research Director of the Japan Science Technology Institute, CREST. Okamura's research group cloned mammalian Period genes, visualized clock oscillation at the single cell level in the central clock of the SCN, and proposed a time-signal neuronal pathway to the adrenal gland. He received a Medal of Honor with Purple Ribbon in 2007 for his research and was awarded Aschoff's Ruler for his work on circadian rhythms in rodents. His lab recently revealed the effects of m6A mRNA methylation on the circadian clock, neuronal communications in jet lag, and the role of dysregulated clocks in salt-induced hypertension.
The Society for Research on Biological Rhythms (SRBR) is an international chronobiological research society with three key goals:
- to promote the advancement and dissemination of basic and applied research in all aspects of biological rhythms.
- to enhance the education and training of students and researchers in the field.
- to foster interdisciplinary communication and an international exchange of ideas.
William Joseph Schwartz is an American neurologist and scientist who serves as Professor and Associate Chair for Research and Education in the neurology department at the University of Texas Dell Medical School. His work on the neurobiology of circadian timekeeping has focused on the mammalian suprachiasmatic nucleus. Schwartz demonstrated that the suprachiasmatic nucleus is rhythmic in vivo using a 2-deoxyglucose radioactive marker for functional brain imaging. As of 2014, he is editor of the Journal of Biological Rhythms.
Robert Y. Moore is an American neurologist with interests in disorders of biological rhythms, movement disorders, and behavioral neurology. He is credited with discovering the function of the suprachiasmatic nucleus (SCN) as the circadian clock, as well as, describing its organization. He is also credited with establishing the role of the mammalian retinohypothalamic tract (RHT) as a photic entrainment pathway. Moore cin 2017 serves as a professor of neurology, with a secondary in psychiatry and neuroscience at the University of Pittsburgh, and as co-director of the National Parkinson Foundation Center of Excellence at the University of Pittsburgh.
Michael Harvey Hastings is a British neuroscientist who works at the Medical Research Council MRC Laboratory of Molecular Biology (LMB) in Cambridge, UK. Hastings is known for his contributions to the current understanding of biological clocks in mammals and marine invertebrates.
Sato Honma is a Japanese chronobiologist who researches the biological mechanisms of circadian rhythms. She mainly collaborates with Ken-Ichi Honma on publications, and both of their primary research focuses are the human circadian clock under temporal isolation and the mammalian suprachiasmatic nucleus (SCN), its components, and associates. Honma is a retired professor at the Hokkaido University School of Medicine in Sapporo, Japan. She received her Ph.D. in physiology from Hokkaido University. She taught physiology at the School of Medicine and then at the Research and Education Center for Brain Science at Hokkaido University. She is currently the director at the Center for Sleep and Circadian Rhythm Disorders at Sapporo Hanazono Hospital and works as a somnologist.
Johanna H. Meijer is a Dutch scientist who has contributed significantly to the field of chronobiology. Meijer has made notable contributions to the understanding of the neural and molecular mechanisms of circadian pacemakers. She is known for her extensive studies of photic and non-photic effects on the mammalian circadian clocks. Notably, Meijer is the 2016 recipient of the Aschoff and Honma Prize, one of the most prestigious international prizes in the circadian research field. In addition to still unraveling neuronal mechanisms of circadian clocks and their applications to health, Meijer's lab now studies the effects of modern lifestyles on our circadian rhythm and bodily functions.
Elizabeth Maywood is an English researcher who studies circadian rhythms and sleep in mice. Her studies are focused on the suprachiasmatic nucleus (SCN), a small region of the brain that controls circadian rhythms.
Ken-Ichi Honma is a Japanese chronobiologist who researches the biological mechanisms underlying circadian rhythms. After graduating from Hokkaido University School of Medicine, he practiced clinical psychiatry before beginning his research. His recent research efforts are centered around photic and non-photic entrainment, the structure of circadian clocks, and the ontogeny of circadian clocks. He often collaborates with his wife, Sato Honma, in work involving the mammalian suprachiasmatic nucleus (SCN), its components, and associated topics.
Martin R. Ralph is a circadian biologist who serves as a professor in the Psychology Department at the University of Toronto. His research primarily focuses on circadian rhythmicity in the fields of neuroscience, psychology, and endocrinology. His most notable work was has been on the suprachiasmatic nucleus, now recognized as the central circadian pacemaker in mammals, but has also investigated circadian rhythms in the context of time, memory, and light.
