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A circadian rhythm is any biological process that displays an endogenous, entrainable oscillation of about 24 hours. These rhythms are driven by a circadian clock, and rhythms have been widely observed in plants, animals, fungi and cyanobacteria. The term circadian comes from the Latin circa, meaning "around" (or "approximately"), and diem or dies, meaning "day". The formal study of biological temporal rhythms, such as daily, tidal, weekly, seasonal, and annual rhythms, is called chronobiology. Although circadian rhythms are endogenous ("built-in", self-sustained), they are adjusted (entrained) to the local environment by external cues called zeitgebers, commonly the most important of which is daylight.


The earliest known account of a circadian process dates from the 4th century BC, when Androsthenes of Thasos, a ship captain serving under Alexander the Great, described diurnal leaf movements of the tamarind tree.[1]

The first recorded observation of an endogenous circadian oscillation was by the French scientist Jean-Jacques d'Ortous de Mairan in 1729. He noted that 24-hour patterns in the movement of the leaves of the plant Mimosa pudica continued even when the plants were kept in constant darkness, in the first experiment to attempt to distinguish an endogenous clock from responses to daily stimuli.[2][3]

In 1896, Patrick and Gilbert observed that during a prolonged period of sleep deprivation, sleepiness increases and decreases with a period of approximately 24 hours.[4] In 1918, J.S. Szymanski showed that animals are capable of maintaining 24-hour activity patterns in the absence of external cues such as light and changes in temperature.[5] Ron Konopka and Seymour Benzer isolated the first clock mutant in Drosophila in the early 1970s and mapped the "period" gene, the first discovered genetic component of a circadian clock.[6] Joseph Takahashi discovered the first mammalian 'clock gene' (CLOCK) using mice in 1994.[7][8]

The term "circadian" was coined by Franz Halberg in the late 1950s.[9]


To be called circadian, a biological rhythm must meet these four general criteria:

  1. The rhythms repeat once a day (they have a 24-hour period). In order to keep track of the time of day, a clock must be at the same point at the same time each day, i.e. repeat every 24 hours.
  2. The rhythms persist in the absence of external cues (endogenous). The rhythm persists in constant conditions with a period of about 24 hours. The rationale for this criterion is to distinguish circadian rhythms from simple responses to daily external cues. A rhythm cannot be said to be endogenous unless it has been tested in conditions without external periodic input.
  3. The rhythms can be adjusted to match the local time (entrainable). The rhythm can be reset by exposure to external stimuli (such as light and heat), a process called entrainment. The rationale for this criterion is to distinguish circadian rhythms from other imaginable endogenous 24-hour rhythms that are immune to resetting by external cues, and hence do not serve the purpose of estimating the local time. Travel across time zones illustrates the ability of the human biological clock to adjust to the local time; a person will usually experience jet lag before entrainment of their circadian clock has brought it into sync with local time.
  4. The rhythms maintain circadian periodicity over a range of physiological temperatures; they exhibit temperature compensation. Some organisms live at a broad range of temperatures, and differences in thermal energy will affect the kinetics of all molecular processes in their cell(s). In order to keep track of time, the organism's circadian clock must maintain a roughly 24-hour periodicity despite the changing kinetics, a property known as temperature compensation.


Template:Refimprove section Photosensitive proteins and circadian rhythms are believed to have originated in the earliest cells, with the purpose of protecting the replicating of DNA from high ultraviolet radiation during the daytime. As a result, replication was relegated to the dark. The fungus Neurospora, which exists today, retains this clock-regulated mechanism.

Circadian rhythms allow organisms to anticipate and prepare for precise and regular environmental changes; they have great value in relation to the outside world. The rhythmicity appears to be as important in regulating and coordinating internal metabolic processes, as in coordinating with the environment.[10] This is suggested by the maintenance (heritability) of circadian rhythms in fruit flies after several hundred generations in constant laboratory conditions,[11] as well as in creatures in constant darkness in the wild, and by the experimental elimination of behavioural but not physiological circadian rhythms in quail.[12][13]

The simplest known circadian clock is that of the prokaryotic cyanobacteria. Recent research has demonstrated that the circadian clock of Synechococcus elongatus can be reconstituted in vitro with just the three proteins of their central oscillator. This clock has been shown to sustain a 22-hour rhythm over several days upon the addition of ATP. Previous explanations of the prokaryotic circadian timekeeper were dependent upon a DNA transcription/translation feedback mechanism.

A defect in the human homologue of the Drosophila "period gene" was identified as a cause of the sleep disorder FASPS (Familial advanced sleep phase syndrome), underscoring the conserved nature of the molecular circadian clock through evolution. Many more genetic components of the biological clock are now known. Their interactions result in an interlocked feedback loop of gene products resulting in periodic fluctuations that the cells of the body interpret as a specific time of the day.

It is now known that the molecular circadian clock can function within a single cell; i.e., it is cell-autonomous.[14] At the same time, different cells may communicate with each other resulting in a synchronised output of electrical signaling. These may interface with endocrine glands of the brain to result in periodic release of hormones. The receptors for these hormones may be located far across the body and synchronise the peripheral clocks of various organs. Thus, the information of the time of the day as relayed by the eyes travels to the clock in the brain, and, through that, clocks in the rest of the body may be synchronised. This is how the timing of, for example, sleep/wake, body temperature, thirst, and appetite are coordinately controlled by the biological clock.

Animal circadian rhythms

Main article: Animal circadian rythms

Circadian rhythms are important in determining the sleeping and feeding patterns of all animals, including humans. There are clear patterns of brain wave activity, hormone production, cell regeneration and other biological activities linked to this 24-hour cycle.

Circadian rhythmicity is present in the sleeping and feeding patterns of animals, including human beings. There are also clear patterns of core body temperature, brain wave activity, hormone production, cell regeneration and other biological activities. In addition, photoperiodism, the physiological reaction of organisms to the length of day or night, is vital to both plants and animals, and the circadian system plays a role in the measurement and interpretation of day length.

Timely prediction of seasonal periods of weather conditions, food availability or predator activity is crucial for survival of many species. Although not the only parameter, the changing length of the photoperiod ('daylength') is the most predictive environmental cue for the seasonal timing of physiology and behavior, most notably for timing of migration, hibernation and reproduction.[15]

Impact of light–dark cycle

The rhythm is linked to the light–dark cycle. Animals, including humans, kept in total darkness for extended periods eventually function with a freerunning rhythm. Their sleep cycle is pushed back or forward each "day", depending on whether their "day", their endogenous period, is shorter or longer than 24 hours. The environmental cues that reset the rhythms each day are called zeitgebers (from the German, "time-givers").[16] It is interesting to note that totally-blind subterranean mammals (e.g., blind mole rat Spalax sp.) are able to maintain their endogenous clocks in the apparent absence of external stimuli. Although they lack image-forming eyes, their photoreceptors (which detect light) are still functional; they do surface periodically as well.[17]

Freerunning organisms that normally have one or two consolidated sleep episodes will still have them when in an environment shielded from external cues, but the rhythm is, of course, not entrained to the 24-hour light–dark cycle in nature. The sleep–wake rhythm may, in these circumstances, become out of phase with other circadian or ultradian rhythms such as metabolic, hormonal, CNS electrical, or neurotransmitter rhythms.[18]

Recent research has influenced the design of spacecraft environments, as systems that mimic the light–dark cycle have been found to be highly beneficial to astronauts.[citation needed]

Arctic animals

Norwegian researchers at the University of Tromsø have shown that some Arctic animals (ptarmigan, reindeer) show circadian rhythms only in the parts of the year that have daily sunrises and sunsets. In one study of reindeer, animals at 70 degrees North showed circadian rhythms in the autumn, winter, and spring, but not in the summer. Reindeer at 78 degrees North showed such rhythms only autumn and spring. The researchers suspect that other Arctic animals as well may not show circadian rhythms in the constant light of summer and the constant dark of winter.[19][20]

However, another study in northern Alaska found that ground squirrels and porcupines strictly maintained their circadian rhythms through 82 days and nights of sunshine. The researchers speculate that these two small mammals see that the apparent distance between the sun and the horizon is shortest once a day, and, thus, a sufficient signal to adjust by.[21]

Butterfly migration

The navigation of the fall migration of the Eastern North American monarch butterfly (Danaus plexippus) to their overwintering grounds in central Mexico uses a time-compensated sun compass that depends upon a circadian clock in their antennae.[22][23]

Human circadian rythms

Human aspects of this area are considered under human biological rythms.

Early research into circadian rhythms suggested that most people preferred a day closer to 25 hours when isolated from external stimuli like daylight and timekeeping. However, this research was faulty because it failed to shield the participants from artificial light. Although subjects were shielded from time cues (like clocks) and daylight, the researchers were not aware of the phase-delaying effects of indoor electric lights.[24] The subjects were allowed to turn on light when they were awake and to turn it off when they wanted to sleep. Electric light in the evening delayed their circadian phase. These results became well-known.[25]

More recent research has shown that: adults have a built-in day, which averages about 24 hours; indoor lighting does affect circadian rhythms; and most people attain their best-quality sleep during their chronotype-determined sleep periods. A study by Czeisler et al. at Harvard found the range for normal, healthy adults of all ages to be quite narrow: 24 hours and 11 minutes ± 16 minutes. The "clock" resets itself daily to the 24-hour cycle of the Earth's rotation.[25]

Circadian rhythm and mental health.

Disturbance of the circadian rhythm is a feature of a number of psychological problems such as depression,PTSD, OCD etc. The significance of the correlation is if any is unclear.

Main article: Mental health and circadian rhythm

Human physical health

Timing of medical treatment in coordination with the body clock may significantly increase efficacy and reduce drug toxicity or adverse reactions. For example, appropriately timed treatment with angiotensin converting enzyme inhibitors (ACEi) may reduce nocturnal blood pressure and also benefit left ventricular (reverse) remodelling.[26]

File:Day Sleepers crop.jpg

A short nap during the day does not affect circadian rhythms.

A number of studies have concluded that a short period of sleep during the day, a power-nap, does not have any measurable effect on normal circadian rhythms, but can decrease stress and improve productivity.[27][28]

There are many health problems associated with disturbances of the human circadian rhythm, such as seasonal affective disorder (SAD), delayed sleep phase syndrome (DSPS) and other circadian rhythm disorders.[29] Circadian rhythms also play a part in the reticular activating system, which is crucial for maintaining a state of consciousness. In addition, a reversal in the sleep–wake cycle may be a sign or complication of uremia,[30] azotemia or acute renal failure.

Studies have also shown that light has a direct effect on human health because of the way it influences the circadian rhythms.[31][32][33][34]

Circadian rhythm and airline pilots

Due to the work nature of airline pilots, who often traverse multiple timezones and regions of sunlight and darkness in one day, and spend many hours awake both day and night, they are often unable to maintain sleep patterns that correspond to the natural human circadian rhythm; this situation can easily lead to fatigue. The NTSB cites this situation as a contributing factor to many accidents[35] and has conducted multiple research studies in order to find methods of combating fatigue in pilots.[36][37]


Further information: Circadian rhythm sleep disorder

Disruption to rhythms usually has a negative effect. Many travellers have experienced the condition known as jet lag, with its associated symptoms of fatigue, disorientation and insomnia.

A number of other disorders, for example bipolar disorder and some sleep disorders, are associated with irregular or pathological functioning of circadian rhythms. Recent research suggests that circadian rhythm disturbances found in bipolar disorder are positively influenced by lithium's effect on clock genes.[38]

Disruption to rhythms in the longer term is believed to have significant adverse health consequences on peripheral organs outside the brain, particularly in the development or exacerbation of cardiovascular disease.[39] The suppression of melatonin production associated with the disruption of the circadian rhythm may increase the risk of developing cancer.[40] LED lighting suppresses melatonin production five times more than a high pressure sodium light.[41] Depression symptoms from long term nighttime light exposure can be undone by returning to a normal cycle.[42]

Effect of drugs

Circadian rhythms and clock genes expressed in brain regions outside the suprachiasmatic nucleus may significantly influence the effects produced by drugs such as cocaine.[43][44] Moreover, genetic manipulations of clock genes profoundly affect cocaine's actions.[45]


It appears that the SCN takes the information on day length from the retina, interprets it, and passes it on to the pineal gland (a pea-like structure found on the epithalamus), which then secretes the hormone melatonin in response. Secretion of melatonin peaks at night and ebbs during the day. The SCN does not appear to be able to react rapidly to changes in the light/dark cues.

Recently, evidence has emerged that circadian rhythms are found in many cells in the body--outside of the SCN "master clock." Liver cells, for example, appear to respond to feeding rather than light. Cells from many parts of the body appear to have "free-running" rhythms.

Disruption to rhythms usually have a negative effect in the short term. Many travelers have experienced the condition known as jet lag, with its associated symptoms of fatigue, disorientation and insomnia. A number of other sleep disorders are associated with irregular or pathological functioning of the circadian rhythms.

Recent research suggest that circadian rhythms and clock genes expressed in brain regions outside the SCN may significantly influence the effects produced by drugs of abuse such as cocaine [1][2]. Moreover, genetic manipulations of clock genes profundly affect cocaine's actions [3].

Circadian rhythms also play a part in the reticular activating system in reticular formation.

Biological clock in mammals

File:Circadian rhythm labeled.jpg

Diagram illustrating the influence of light and darkness on circadian rhythms and related physiology and behaviour through the suprachiasmatic nucleus in humans.

The primary circadian "clock" in mammals is located in the suprachiasmatic nucleus (or nuclei) (SCN), a pair of distinct groups of cells located in the hypothalamus. Destruction of the SCN results in the complete absence of a regular sleep–wake rhythm. The SCN receives information about illumination through the eyes. The retina of the eye contains "classical" photoreceptors ("rods" and "cones"), which are used for conventional vision. But the retina also contains specialized ganglion cells which are directly photosensitive, and project directly to the SCN where they help in the entrainment of this master circadian clock.

These cells contain the photopigment melanopsin and their signals follow a pathway called the retinohypothalamic tract, leading to the SCN. If cells from the SCN are removed and cultured, they maintain their own rhythm in the absence of external cues.

The SCN takes the information on the lengths of the day and night from the retina, interprets it, and passes it on to the pineal gland, a tiny structure shaped like a pine cone and located on the epithalamus. In response, the pineal secretes the hormone melatonin. Secretion of melatonin peaks at night and ebbs during the day and its presence provides information about night-length.

Several studies have indicated that pineal melatonin feeds back on SCN rhythmicity to modulate circadian patterns of activity and other processes. However, the nature and system-level significance of this feedback are unknown.

The circadian rhythms of humans can be entrained to slightly shorter and longer periods than the Earth's 24 hours. Researchers at Harvard have recently shown that human subjects can at least be entrained to a 23.5-hour cycle and a 24.65-hour cycle (the latter being the natural solar day-night cycle on the planet Mars).[46]

Biological markers

The classic phase markers for measuring the timing of a mammal's circadian rhythm are:

For temperature studies, subjects must remain awake but calm and semi-reclined in near darkness while their rectal temperatures are taken continuously. Template:Reference necessary, though variation is great among normal chronotypes.

Melatonin is absent from the system or undetectably low during daytime. Its onset in dim light, dim-light melatonin onset (DLMO), at about 21:00 (9 p.m.) can be measured in the blood or the saliva. Its major metabolite can also be measured in morning urine. Both DLMO and the midpoint (in time) of the presence of the hormone in the blood or saliva have been used as circadian markers. However, newer research indicates that the melatonin offset may be the more reliable marker. Benloucif et al. in Chicago in 2005 found that melatonin phase markers were more stable and more highly correlated with the timing of sleep than the core temperature minimum. They found that both sleep offset and melatonin offset were more strongly correlated with the various phase markers than sleep onset. In addition, the declining phase of the melatonin levels was more reliable and stable than the termination of melatonin synthesis.[47]

One method used for measuring melatonin offset is to analyse a sequence of urine samples throughout the morning for the presence of the melatonin metabolite 6-sulphatoxymelatonin (aMT6s). Laberge et al. in Quebec in 1997 used this method in a study that confirmed the frequently found delayed circadian phase in healthy adolescents.[49]

A third marker of the human pacemaker is the timing of the maximum plasma cortisol level. Klerman et al. in 2002 compared cortisol and temperature data to eight different analysis methods of plasma melatonin data, and found that "methods using plasma melatonin data may be considered more reliable than methods using CBT or cortisol data as an indicator of circadian phase in humans."[48]

Other physiological changes which occur according to a circadian rhythm include heart rate and production of red blood cells.[50]

Outside the "master clock"

More-or-less independent circadian rhythms are found in many organs and cells in the body outside the suprachiasmatic nuclei (SCN), the "master clock". These clocks, called peripheral oscillators, are found in the oesophagus, lungs, liver, pancreas, spleen, thymus, and the skin.[51] Though oscillators in the skin respond to light, a systemic influence has not been proven so far.[52][53] There is also some evidence that the olfactory bulb and prostate may experience oscillations when cultured, suggesting that these structures may also be weak oscillators.

Furthermore, liver cells, for example, appear to respond to feeding rather than to light. Cells from many parts of the body appear to have freerunning rhythms.

Light and the biological clock

Further information: Light effects on circadian rhythm

Light resets the biological clock in accordance with the phase response curve (PRC). Depending on the timing, light can advance or delay the circadian rhythm. Both the PRC and the required illuminance vary from species to species and lower light levels are required to reset the clocks in nocturnal rodents than in humans.

Lighting levels that affect the circadian rhythm in humans are higher than the levels usually used in artificial lighting in homes. According to some researchers[54] the illumination intensity that excites the circadian system has to reach up to 1000 lux striking the retina.

In addition to light intensity, wavelength (or colour) of light is a factor in the entrainment of the body clock. Melanopsin is most efficiently excited by light from the blue part of the spectrum (420–440 nm[55] according to some researchers while others have reported 470–485 nm). These blue wavelengths are present in virtually all light sources, therefore their elimination requires special lights or filters which appear amber.

It is thought that the direction of the light may have an effect on entraining the circadian rhythm;[54] light coming from above, resembling an image of a bright sky, has greater effect than light entering our eyes from below.

According to a 2010 study completed by the Lighting Research Center, daylight has a direct effect on circadian rhythms and, consequently, on performance and well-being. The research showed that students who experience disruption in lighting schemes in the morning consequently experience disruption in sleeping patterns. The change in sleeping patterns may lead to negatively impacted student performance and alertness. Removing circadian light in the morning delays the dim light melatonin onset by 6 minutes a day, for a total of 30 minutes for five days.[56]

Enforced longer cycles

Studies by Nathaniel Kleitman[57] in 1938 and by Derk-Jan Dijk and Charles Czeisler[58][59] in 1994/5 have put human subjects on enforced 28-hour sleep–wake cycles, in constant dim light and with other time cues suppressed, for over a month. Because normal people cannot entrain to a 28-hour day[60] in dim light if at all, this is referred to as a forced desynchrony protocol. Sleep and wake episodes are uncoupled from the endogenous circadian period of about 24.18 hours and researchers are allowed to assess the effects of circadian phase on aspects of sleep and wakefulness including sleep latency and other functions.[61]

See also



  1. Bretzl, H. (1903). Botanische Forschungen des Alexanderzuges, Leipzig: Teubner.Template:Page needed
  2. de Mairan JJO (1729). Observation Botanique. Histoire de l'Academie Royale des Sciences: 35–36.
  3. Gardner MJ, Hubbard KE, Hotta CT, Dodd AN, Webb AA (July 2006). How plants tell the time. The Biochemical Journal 397 (1): 15–24.
  4. Dijk, Derk-Jan (August 2005). Timing and Consolidation of Human Sleep, Wakefulness, and Performance by a Symphony of Oscillators. J Biol Rhythms 20 (4): 279–290.
  5. Danchin, Antoine. Important dates 1900–1919. HKU-Pasteur Research Centre.
  6. Konopka R, Benzer S (September 1971). Clock Mutants of Drosophila melanogaster. Proc. Nat. Acad. Sci. USA 68 (9): 2112–2116.
  7. includeonly>"Gene Discovered in Mice that Regulates Biological Clock", Chicago Tribune, 29 April 1994.
  8. Vitaterna MH (April 1994). Mutagenesis and mapping of a mouse gene, Clock, essential for circadian behavior. Science 264 (5159): 719–25.
  9. Zivkovic, Bora Circadian Quackery. A Blog Around the Clock. ScienceBlogs. URL accessed on 2010-02-23.
  10. Sharma, V.K. (November 2003). Adaptive significance of circadian clocks. Chronobiology International 20 (6): 901–19.
  11. Sheeba, V.; Sharma, V.K.; Chandrashekaran, M.K.; Joshi, A. (September 1999). Persistence of eclosion rhythm in Drosophila melanogaster after 600 generations in an aperiodic environment. Die Naturwissenschaften 86 (9): 448–9.
  12. Guyomarc'h, C.; Lumineau, S.; Richard, J.P. (May 1998). Circadian rhythm of activity in Japanese quail in constant darkness: variability of clarity and possibility of selection. Chronobiology International 15 (3): 219–30.
  13. Zivkovic, B.D.; Underwood, H.; Steele, C.T.; Edmonds, K. (October 1999). Formal properties of the circadian and photoperiodic systems of Japanese quail: phase response curve and effects of T-cycles. Journal of Biological Rhythms 14 (5): 378–90.
  14. Nagoshi, E.; Saini, C.; Bauer, C.; Laroche, T.; Naef, F.; Schibler, U. (November 2004). Circadian gene expression in individual fibroblasts: cell-autonomous and self-sustained oscillators pass time to daughter cells. Cell 119 (5): 693–705.
  15. Zivkovic, Bora "Coturnix" Clock Tutorial #16: Photoperiodism - Models and Experimental Approaches. A Blog Around the Clock. ScienceBlogs. URL accessed on 2007-12-09.
  16. Shneerson, J.M.; Ohayon, M.M.; Carskadon, M.A. (2007). Circadian rhythms. Rapid eye movement (REM) sleep. Armenian Medical Network. URL accessed on 2007-09-19.
  17. "The Rhythms of Life: The Biological Clocks That Control the Daily Lives of Every Living Thing" Russell Foster & Leon Kreitzman, Publisher: Profile Books Ltd.
  18. Regestein, Quentin R. (September 1995). Treatment of delayed sleep phase syndrome. General Hospital Psychiatry 17 (5): 335–345.
  19. includeonly>Spilde, Ingrid. "Reinsdyr uten døgnrytme",, December 2005. Retrieved on 2007-11-24. (in Norwegian, Bokmål)
  20. Zivkovic, Bora, aka Coturnix, chronobiologist Circadian Rhythms, or Not, in Arctic Reindeer. A Blog around the Clock. URL accessed on 2007-11-24.
  21. Zivkovic, Bora, aka Coturnix, chronobiologist. Small Arctic Mammals Entrain to Something during the Long Summer Day. A Blog Around the Clock. URL accessed on 2007-11-26.
  22. Merlin, C.; Gegear, R.J.; Reppert, S.M. (September 2009). Antennal circadian clocks coordinate sun compass orientation in migratory monarch butterflies. Science 325 (5948): 1700–4.
  23. Kyriacou, C.P. (September 2009). Physiology. Unraveling traveling. Science 325 (5948): 1629–30.
  24. Duffy, Jeanne F., Kenneth P. Wright, Jr. (August 2005). Entrainment of the Human Circadian System by Light (Review). J Biol Rhythms 20 (4): 326–338.
  25. 25.0 25.1 Charles A. Czeisler MD, PhD (1999). Human Biological Clock Set Back an Hour. URL accessed on 2007-09-23.
  26. Grote L, Mayer J, Penzel T, Cassel W, Krzyzanek E, Peter JH, von Wichert P (1994). Nocturnal hypertension and cardiovascular risk: consequences for diagnosis and treatment. J Cardiovasc Pharmacol 24 (Suppl 2): S26–38.
  27. Pilcher, J.J.; Michalowski, K.R.; Carrigan, R.D. (2001). The prevalence of daytime napping and its relationship to nighttime sleep. Behavioral Medicine 27 (2): 71–6.
  28. Emily Rolston, Judy R. Sandlin, Michael Sandlin, and Rosanne Keathley (2007). Power-Napping: Effects on Cognitive Ability and Stress Levels Among College Students. Power-Napping: Effects on Cognitive Ability and Stress Levels Among College Students. Liberty University. URL accessed on 2008-11-11.
  29. Sabah Quraishi (2007). Circadian Rhythms and Sleep. Circadian Rhythms and Sleep. Serendip. URL accessed on 2007-09-19.
  30. Sinert, Richard, Peter R Peacock, Jr Renal Failure, Acute. eMedicine from WebMD. URL accessed on 2008-08-03.
  31. Figueiro, M.G.; Bierman, A.; Plitnick, B.; Rea, M.S. (2009). Preliminary evidence that both blue and red light can induce alertness at night. BMC Neuroscience 10.
  32. Figueiro, M.G.; Rea, M.S.; Bullough, J.D. (2006). Does architectural lighting contribute to breast cancer?. Journal of Carcinogenesis 5: 20.
  33. Sloane, P.D.; Figueiro, M.G.; Cohen, L (March 2008). Light Therapy for Sleep Disorders and Depression in Older Adults. Clinical Geriatrics: 2–8.
  34. New Discovery: Prehistoric Body Clock in Humans Same as That in Algae. The Daily Galaxy.
  35. Aviation Week Article
  36. Pilot Fatigue Study
  37. CNN Article
  38. Yin, L.; Wang, J.; Klein, P.S.; Lazar, M.A. (February 2006). Nuclear receptor Rev-erbalpha is a critical lithium-sensitive component of the circadian clock. Science 311 (5763): 1002–5.
  39. Martino, T.A. (May 2008). Circadian rhythm disorganization produces profound cardiovascular and renal disease in hamsters. American Journal of Physiology. Regulatory, Integrative and Comparative Physiology 294 (5): R1675–83.
  40. Straif, K. (December 2007). Carcinogenicity of shift-work, painting, and fire-fighting. The Lancet Oncology 8 (12): 1065–6.
  41. Dangers of exposure to 'white' light. University of Haifa. URL accessed on 24 July 2012.
  42. SOME HARMFUL EFFECTS OF LIGHT AT NIGHT CAN BE REVERSED, STUDY FINDS. Tracy Bedrosian and Randy Nelson. Ohio State University. URL accessed on 24 July 2012.
  43. Uz, T.; Akhisaroglu, M.; Ahmed, R.; Manev, H. (December 2003). The pineal gland is critical for circadian Period1 expression in the striatum and for circadian cocaine sensitization in mice. Neuropsychopharmacology 28 (12): 2117–23.
  44. Kurtuncu, M.; Arslan, A.D.; Akhisaroglu, M.; Manev, H.; Uz, T. (April 2004). Involvement of the pineal gland in diurnal cocaine reward in mice. European Journal of Pharmacology 489 (3): 203–5.
  45. McClung, C.A. (June 2005). Regulation of dopaminergic transmission and cocaine reward by the Clock gene. Proceedings of the National Academy of Sciences of the United States of America 102 (26): 9377–81.
  46. Scheer, F.A.; Wright, K.P.; Kronauer, R.E.; Czeisler, C.A. (2007). Plasticity of the intrinsic period of the human circadian timing system. PLoS ONE 2 (1): e721.
  47. 47.0 47.1 Benloucif, S.; Guico, M.J.; Reid, K.J.; Wolfe, L.F.; L'hermite-Balériaux, M.; Zee, P.C. (April 2005). Stability of melatonin and temperature as circadian phase markers and their relation to sleep times in humans. Journal of Biological Rhythms 20 (2): 178–88.
  48. 48.0 48.1 Klerman, Elizabeth B., Hayley B. Gershengorn, Jeanne F. Duffy, Richard E. Kronauer (April 2002). Comparisons of the Variability of Three Markers of the Human Circadian Pacemaker. J Biol Rhythms 17 (2): 181–193.
  49. Laberge, L., Lesperance, P.; Tremblay, R.; Lambert, C.; Montplaisir, J. (1997). Phase delay of 6-sulphatoxymelatonin in normal adolescents. Sleep Research 26.
  50. Volicer L, Harper DG, Manning BC, Goldstein R, Satlin A (May 2001). Sundowning and circadian rhythms in Alzheimer's disease. Am J Psychiatry 158 (5): 704–11.
  51. Zanello, S.B.; Jackson, D.M.; Holick, M.F. (October 2000). Expression of the circadian clock genes clock and period1 in human skin. The Journal of Investigative Dermatology 115 (4): 757–60.
  52. Kawara, S. (December 2002). Low-dose ultraviolet B rays alter the mRNA expression of the circadian clock genes in cultured human keratinocytes. The Journal of Investigative Dermatology 119 (6): 1220–3.
  53. Campbell, S.S.; Murphy, P.J. (January 1998). Extraocular circadian phototransduction in humans. Science 279 (5349): 396–9.
  54. 54.0 54.1 Semjonova, Milena (2003). Healthy Lighting, from a lighting designer's perspective. Milena Lighting Design.
  55. Newman, L.A.; Walker, M.T.; Brown, R.L.; Cronin, T.W.; Robinson, P.R. (November 2003). Melanopsin forms a functional short-wavelength photopigment. Biochemistry 42 (44): 12734–8.
  56. Figueiro, M.G.; Rea, M.S. (February 2010). Lack of short-wavelength light during the school day delays dim light melatonin onset (DLMO) in middle school students. Neuro Endocrinology Letters 31 (1): 92–6.
  57. Kleitman, Nathaniel (1962). Sleep and Wakefullness ed 2, Chicago: University of Chicago Press.
  58. Dijk, D.J.; Czeisler, C.A. (January 1994). Paradoxical timing of the circadian rhythm of sleep propensity serves to consolidate sleep and wakefulness in humans. Neuroscience Letters 166 (1): 63–8.
  59. Dijk, D.J.; Czeisler, C.A. (May 1995). Contribution of the circadian pacemaker and the sleep homeostat to sleep propensity, sleep structure, electroencephalographic slow waves, and sleep spindle activity in humans. The Journal of Neuroscience 15 (5 Pt 1): 3526–38.
  60. Cromie, William J. Human Biological Clock Set Back an Hour. The Harvard University Gazette. URL accessed on 2008-02-19.
  61. Aldrich, Michael S. (1999). Sleep medicine, New York: Oxford University Press.

Further reading

Key texts


  • Aschoff, J. (ed.) (1965) Circadian Clocks. North Holland Press, Amsterdam
  • Dunlap, J.C.; Loros, J.; DeCoursey, P.J. (2003) Chronobiology: Biological Timekeeping. Sinauer, Sunderland
  • Koukkari, W.L.; Sothern, R.B. (2006) Introducing Biological Rhythms. Springer, New York


  • Akerstedt, T. (1985) Adjustment of physiological circadian rhythms and the steep-wake cycle to shiftwork. In: S. Folkard and T.H. Monk (eds) Hours of Work, Chichester: John Wiley.
  • Avivi, A.; Albrecht, U.; Oster, H.; Joel, A.; Beiles, A.; Nevo, E. (November 2001). Biological clock in total darkness: the Clock/MOP3 circadian system of the blind subterranean mole rat. Proceedings of the National Academy of Sciences of the United States of America 98 (24): 13751–6.
  • Avivi, A.; Oster, H.; Joel, A.; Beiles, A.; Albrecht, U.; Nevo, E. (September 2002). Circadian genes in a blind subterranean mammal II: conservation and uniqueness of the three Period homologs in the blind subterranean mole rat, Spalax ehrenbergi superspecies. Proceedings of the National Academy of Sciences of the United States of America 99 (18): 11718–23.
  • Ditty, J.L.; Williams, S.B.; Golden, S.S. (2003). A cyanobacterial circadian timing mechanism. Annual Review of Genetics 37: 513–43.
  • Dvornyk, V.; Vinogradova, O.; Nevo, E. (March 2003). Origin and evolution of circadian clock genes in prokaryotes. Proceedings of the National Academy of Sciences of the United States of America 100 (5): 2495–500.
  • Martino, T. (April 2004). Day/night rhythms in gene expression of the normal murine heart. Journal of Molecular Medicine 82 (4): 256–64.
  • Refinetti, R. (2006) Circadian Physiology, 2nd ed. CRC Press, Boca Raton
  • Takahashi, J.S.; Zatz, M. (September 1982). Regulation of circadian rhythmicity. Science 217 (4565): 1104–11.
  • Tomita, J.; Nakajima, M.; Kondo, T.; Iwasaki, H. (January 2005). No transcription-translation feedback in circadian rhythm of KaiC phosphorylation. Science 307 (5707): 251–4.
  • Moore-Ede, Martin C. (1982). The Clocks that Time Us: Physiology of the Circadian Timing System, Cambridge, Massachusetts: Harvard University Press.

Additional material



  • Google Scholar
  • Aschoff, J. (1965) Circadian rhythms of a Russian vocabulary, Journal of Experimental Psychology: Human Learning and Memory 104: 126-33.
  • Czeisler, C.A., Moore-Ede, M.C. and Coleman, R.M. (1982) Rotating shift work schedules that disrupt sleep are improved by applying circadian principles, Science 217: 460-3.
  • Czeisler, C.A., Weizman, E.D., Moore-Ede, M.C., Zimmerman, J.C. and Knauer, R.S. (1980) Human sleep: its duration and organisation depend on its circadian phase, Science 210: 1264-7.
  • Folkard, S. (1983) Circadian rhythms and hours of work. In: P. Want (ed.) Psychology at Work, Harmondsworth: Penguin.
  • Miles, L.E., Raynal, D.M. and Wilson, M.A. (1977)Blind man living in normal society has circadian rhythm of 24.9 hours, Science 198: 421-3.

External links

Biological Clocks A description of circadian rhythms in plants by de Mairan, Linnaeus, and Darwin Stanford info page Journal of Circadian Rhythms

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