The Atlas of the Biological Rhythmicities

A Survey of Research in Biological Rhythmicity

By Ahad Ghorbani



The Atlas of the Biological Rhythmicities. 1

A Survey of Research in Biological Rhythmicity. 1

Application of Biological Rhythms. 20

Human Health. 20

Glossary. 21

A. Human’s Rhythms. 29

A1. Ultradian Rhythms. 29

A2. Circadian Rhythms. 29

A3. Infradian Rhythms. 30

B. Environmental Cues. 31




The rhythms are a fundamental property of existence. There is rhythmicity in both nature and organisms. There are many different natural, biological and physiological phenomena that show rhythmicity. Biological rhythms are a fundamental element of living organism. Organisms ranging from unicellular algae to human display biological rhythms reflecting their ability at the molecular level to keep track of time. A very fundamental question is if the rhythmicity in the organisms is a passive response to rhythmicity in the nature or the organism has an internal clock or oscillator and can measure the time itself.

Observations and experiments show that organisms are able to measure the time itself. Biological clocks are the physiological systems that measure time in an organism. Research on a range of organisms is revealing an increasingly coherent picture of the molecular and cellular basis of biological clock and the internal pacemakers which generate the rhythms and are synchronised with time cues in the environment such as the light-dark cycle, physical activity, drugs, diet and social factor.

Biological rhythms are controlled by internal biological clocks with another word, by an internal undamped oscillator mechanism as opposed to being passively driven by daily environmental variations. The clocks’ periodicity closely harmonizes with the environmental periodicity. This match allows the biological rhythm to be coupled to the environment cycle in a manner useful and advantageous to the organism. For example, a fruit fly, Drosophila, emerges from its pupal case (cocoon) in early morning when the humidity is high and there is a moderate degree of wetness in the atmosphere. This is absolutely needed to allow its wings to open out gradually and properly from a folded state. Flies that are deceived in the laboratory into emerging in the afternoon have a much lower survival rate than those, which emerge in the morning.

Thus, biological clocks are the physiological systems that enable organisms to live in harmony with the rhythms of nature, such as the rotational cycles of Earth, Moon and Sun. There are such biological timers for nearly every kind of periodicity for all plants, animals, and human. The most of our knowledge comes from the research and study of circadian, or daily, rhythms.

This book will focus on current understanding of the nature and mechanisms of biological rhythmicity in the different organisms. The circadian rhythms in humans and its implication for human health and well-being will be emphasized.

If one will generalize the research of biological rhythms, one can distinguish two different area of study. In one area, rhythmic processes themselves are studied. Researchers try to understand and model the temporal organization. In another area, the nature of biological oscillators and the generation of rhythms are studied. Researchers try to discover how the clocks are regulated, where it is located, and which elements have a biological clock. This book will briefly reflect these two distinct areas.


The Rhythmicities in the Nature

Life on Earth evolved in a periodic universe. All organisms live in this periodic environment. Each organism lives in an ecological sys­tem. Even the most stable ecological system has significant temporal changes in environmental variables such as light and temperature. These variations may result in major differences in environmental conditions. Some of these environmental variations are random, but others are regular.

The regular fluctuations, or oscillations, in environmental conditions result from the periodic motion of planets, such as the daily rotation of the Earth upon its axis, the monthly revolution of the Moon around the Earth, and the annual revolution of the Earth around the sun. These daily, tidal, lunar, and annual cycles result in highly predictable and recurrent changes in the Earth’s environment.

Earth’s Orbital Motion

The Earth’s orbit around the Sun is an ellipse, with Sun at one focus. Perihelion, or the closet approach to Sun, occurs about January 3, and aphelion, farthest from the Sun, about July 4. The Earth’s orbital period is the year. It is exactly 356 days 5 hours 48 minutes 46 seconds.

Figure 1: The earth’s revolution about the sun. The North Pole is tilted toward the sun from March to September, so the northern hemisphere receives more sunlight per day than the southern hemisphere. Then from September to March the southern hemisphere receives more sunlight per day than the northern hemisphere because the North Pole is tilted away from the sun. In midsummer at the North Pole there is continuous light, and in midwinter there is continuous darkness. In contrast, at the equator there is no seasonal change; there are always 12 hours of light and 12 hours of darkness. In the regions between the equator and the poles the duration of the light period continuously changes during the year. (From Moore-Ede et al., 1982)

As the Earth moves in its orbit around the Sun, its north spin axis, or geographic pole, points in the direction of the star Polaris, making it the North Star or polestar. One obvious result is that different parts of the Earth receive differing amount of sunlight; this is the primary cause of seasons.

Earth’s Rotational Motion

The Earth’s period of rotation is the day. The rotation speed is maximum in late July and early August, and minimum in April; the difference in the length of the day is about 0.0012 s. Since about 1900 the Earth’s rotation has been slowing at a rate of about 1.7 s per year. In geologic past the Earth’s rotational period was much faster. Daily, monthly, tidal and annual growth rings on fossil marine organisms reveal that about 350x106 years ago (Middle Devonian Period) the year had 400-410 days, and 280x106 years ago (Pennsylvanian Period) the years had 390 days.

These observations are very close to the calculated value of the effect of tidal friction. The slowing of the Earth’s rotation is caused by tidal friction between the sea floor and the ocean water. The Moon is the main cause of tides, and the total rotational energy in Earth-Moon system is conserved, so that the energy lost by the Earth is gained by the Moon. This causes the Moon to move farther from Earth, and this in turn lengthens the period of the Moon’s revolution.







Figure 2: The Earth’s rotation. As the earth rotates in an easterly direction, the earth’s shadow and the time of dawn and dusk move in a westerly direction across the earth’s surface. The earth’s shadow moves most quickly at the equator and most slowly at the poles, simply because the earth’s circumference is grater at equator so that the shadow has a greater distance to travel. The true period of rotation of the earth is not 24.0 hours, but 23 hours and 56 minutes. But because the earth is revolving about the sun in the same direction that it is rotating, it must turn for an extra 4 minutes each day to complete a rotation. (From Moore-Ede et al., 1982)

Moon’s Revolution

The Moon’s period of revolution and rotation is 27 days 7 hours 43 minutes 11.5 seconds. The orbital plane of the Earth-Moon system is inclined to the orbital plane of the Earth-Sun system at an angle of the tide-raising power of the moon upon the seas of the earth is 2.2 times the tidal influence of the sun.

The organisms modify their internal physiology and behaviour in a way that are in agreement with these changes. That is the organisms measure the time and benefit this time measuring.

Examples of biological rhythmicity that correspond with environmental periodicities are:

      daily rhythms of sleep and wakefulness in humans,      tidal rhythms of plankton migration,       lunar rhythms of reproductive behaviour in invertebrates, lunar rhythms in marine animals living in tidal zones,       and annual rhythms of flowering in plants and reproduction in animals.


The Rhythmicities in the Plants

Physiologists recognized endogenous or persistent rhythmicity in the plants long ago. Examples are daily rhythms of cell division and bioluminenescence in unicellular forms, and of sleep and movement and growth rate in higher forms. Moreover, seeds exhibit an annual rhythm in capacity to germinate.

Leaf movement

The study of leaf movement shows one of the most noticeable charac­teristics of many daily rhythms: when the plant was brought into the laboratory and kept in constant darkness and at constant temperature so that it received no information from the environment about the time of day, the rhythm of leaf movement persisted with a periodicity of about 24 hours. For the first time in 1729, a French astronomer, Jean Jacques d’Ortopus de Mairan, by a critical experiment, shows the endogenous circadian rhythms in plants. He reported to L’Academie Royale des Sciences in Paris, the persistence of daily rhythms of a plant’s leaf movements in a constant condition.

Under these conditions, however, the periodicity does not remain exactly 24 hours. The biological rhythm slowly drifts out of phase with the natural environmental cycle, to say day-night cycle. When the plant is returned to natural conditions, it once again becomes syn­chronized with the natural day-night cycle.

These two properties:

       persistence of the rhythm in constant        The ability to be synchronized to light-dark cycles

are the most important evidence that the observed biological rhythm is under control of an internal undamped oscillator mechanism, rather than simply the response of the organism to immediate changes in the environment, such as light or temperature cycles.

Moreover, flowers open and close and open again at a period typically 1 to 2 hours different from their entrained period when exposed to rising and setting sun.

The Rhythmicities in the Microorganisms

Circadian rhythms are found in lower animals, including microorganisms. Spore formation in fungi is known to be regulated by circadian rhythms. Seawater glows and fades with the 23-hour bioluminencece of swarms of single-celled organisms, each of which also chooses its time by the same internal clock for fission into two cells.

The dinoflagellate Gonyaulax polyedra is a unicellular, which its bioluminescence, cell division, photosynthesis and spontaneous glow occur according a circadian temporal program. These four phenomena happen at noticeable different times in a healthy cell population

Figure 3: In Gonyaulax polyedra, four different circadian rhythms have the same phase-response curve. It means they are driven by a common pacemaker. [J. Aschoff 1981]

Euglena shows a circadian rhythm in phototaxis, cell division and photosynthesis.

Paramecium aurelia shows a temperature-compensated circadian rhythm in frequency of mating reactivity, pairing, and cell division. Neurospora shows a circadian rhythm in condition, CO2 evolution, DNA synthesis and RNA synthesis.

The Rhythmicities in the Invertebrates

Circadian rhythms are found widely in invertebrates, including marines and insects.

Colour changes in fiddler crabs are known to be regulated by circadian rhythms.

Metamorphosing insets choose their moment to emerge as adults as though triggered by an accurate alarm clock. Their biological rhythms are phased to the most “appropriate” time of day. For exam­ple, fruit flies emerge from their pupal case in the early morning when the environmental conditions are the most suitable for their survivance.

Cockroach, Leucophaea maderae, shows a circadian rhythm in locomotor activity and adult emergence (eclosion). Evidence in cockroaches shows a bilaterally paired pacemaker that are mutually coupled.

In the eyes of the mollusk, Aplysia, its pacemaker function is restricted. It shows circadian rhythms in spontaneous neural activity of the optic nerve and locomotory activity.


The Rhythmicities in the Vertebrates

Most of our knowledge about biological rhythmicity is based on ani­mal research, and most of the advances in this field come from animal study.

Circadian rhythms in Birds


Birds, Locomotory activity, feeding


The Rhythmicities in the Mammalians

In the mammalian circadian system, the suprachiasmatic nucleus (SCN) acts as primary pacemaker tuning several hormonal and behavioural rhythms to the environmental light dark cycle.

Any healthy mammal’s heart beats spontaneously, requiring no external pacemaker. The pacemaker cells of the mammalian heart have an intrinsic rhythm of electrical activity, with cycles lasting several 100 ms.

Neural activity in a certain mammalian brain centre (the suprachiasmatic nucleus) waxes and wanes sinusoidally more than 10-fold every 24.5 hours (in rat)

Mammal’s locomotory activity is an endogenous rhythm. A golden hamster is kept in constant darkness and constant temperature. It demonstrates a precise rhythm of free-running activity, with a period (t) approximately 24.4 hours.

Mammals feeding

Figure 4: Free-running rhythm of wheel-running activity in a golden hamster housed in constant light (LL). This record is made by dividing a continuous activity record into 24-hour periods and successive days plotted under each other. Activity periods are shown by bold marks.


The Rhythmicities in the Human

In humans, the circadian clock signals when to sleep and wake, adjusts body temperature and modifies cardiovascular, renal, digestive and other functions, correspondingly directing the production of certain hormones (melatonin).

Human physiology is governed by rhythms and periodic processes. These rhythmicities influence every level of human biology, from the biochemical to the behavioural.


The Human Circadian Rhythmicities

The circadian rhythm refers to self-sustained biological rhythm, which in the organism’s natural environment is normally entrained to a 24- hour period.

The internal undamped oscillator that controls the circadian biological rhythm is called a biological clock, or more specifically, a circadian clock. The term circadian emphasizes that the biological periodicity is approximately equal to 24 hours.

Circadian rhythms have been observed in nearly every major group of organisms except bacteria and blue-green algae.

Melatonin: High levels of melatonin are found during the dark and low levels during the light period of the day.


Table 1: Examples of circadian rhythms in various organisms. (Feldman J, 199x)






Bioluminescence, cell division, photosynthesis


Phototaxis, cell division, photosynthesis


Mating reactivity, cell division


Conidiation, CO2 evolution, DNA synthesis, RNA synthesis




Locomoto activity

          Fruit fly

Adult emergence (eclosion), locomotor activity


Spontaneous neural activity of the optic nerve, locomotory activity




Locomotory activity, feeding


Locomotory activity, feeding


Sleep, body temperature, drug sensitivity, urine excretion, cortisol secretion from adrenal gland, melatonin production, physical or mental performance


It seems that organisms have adapted genetically to the persistently reliable period of the Earth’s rotation, Earth’s revolution, Moon’s revolution and all that goes with them, by developing spontaneous internal “clocks”, which are readily entrained by external influences of about a 24-hour period; thus the adjective “circa” (roughly, about) + “dies” (a day) that is, circadian coined by Frans Halberg in 1959 by combination of these two Latin words.

The observation which permit strongest inference physiological clocks’ existence are taken under conditions of “temporal isolation” from environmental cues (mainly light, temperature, and activities of other organism) of diurnal period- for example, in a deep cave or laboratory simulation of such constancy. Under such conditions: human body temperature rises and falls about one degree Celsius with nearly a 25-kour period; in sleep and alternate- generally with a period of about 25 hours (depending upon the individual).

Figure 5: Circadian rhythms of wakefulness and sleep (solid and open bars, respectively) and rectal temperature (triangles above bars for maxima, below bars for minima) recorded in a subject living in an isolation unit under constant conditions. The free- running period of this subject (t) is 25.3 hours. [From J. Aschoff, Ergonomics 39, 1978]

No circadian clock’s mechanism has yet been deciphered, but the circadian rhythm in humans shows a vast number of functions at the biochemical, physiological, and behavioural levels.

There are mutants for clock period, their genetic loci have been mapped, and in one case (the fruit fly Drosophila) the gene involved has been sequenced. The period can also be adjusted by chemical or pharmaceutical agents. All this suggests that clock mechanisms are basically chemical.

For understanding the mechanisms underlying circadian clocks, the studies have focused on two levels of understanding:

       biochemical       physiological 

Cellular mechanisms of circadian rhythms

Efforts to understand the biochemical or molecular basis of circadian rhythms have met with great difficulties. No single biochemical component of actual clock mechanism has yet been identified in any organism.

A first biochemical approach has been to identify inhibitors, which do alter the timing process in order to ultimately identify the cellular target of the inhibitor and thus obtain a clue about which biochemical events are important for circadian timing. A large number of such inhibitors have been identified, including those, which inhibit protein synthesis, membrane structure, ion transport, and oxidative respiratory functions. Once again, these experiments have not proved very useful in identifying specific clock reactions, because metabolic reactions are so interconnected that one cannot determine whether the clock was altered as a result of the primary target of the inhibitor or as a secondary consequence of that initial condition.

In the second approach, geneticists have isolated mutants, whose cir­cadian clock is altered in some manner. Commonly, these mutants have circadian clocks, which run too fast or too slowly. The inheritance of these clock alternations by the next generation has shown that the aberration is the result of a change in the DNA of a single gene in these individuals. Since each gene in an organism codes for a single protein, one can conclude that in these mutant individuals a change in the structure of one specific protein has resulted in a specific change in their circadian clocks.

With further development of recombinant DNA techniques, it should be possible to clone these specific clock genes and to identify the function of the specific protein for which they code. This would then be the first clear evidence for the role of a specific biochemical event in the functioning of a circadian clock.

Neural Basis and Physiological Mechanisms of Circadian Rhythms

Physiological studies in animals have focused on efforts to determine the location of the master clock, or controlling oscillator, in a multicellular individual. In invertebrates such as cockroackes, experiments involving ablation, or cutting, of different parts of central nervous system have shown that a portion of the brain known as the optic lobes is essential for maintaining circadian rhythmicity. Cockroaches with lesions in both the right and left optic lobes lose all evidence of their circadian rhythm of locomotor activity.

In fruit flies, the brain has also been implicated as a master clock. In mutants that have lost their rhythmicity, a small group of neurosecretory cells in a portion of the brain known as the pars intercerebrallis show an altered morphology. Such cells may secrete a hormone, which controls the daily activity pattern of the fly. Further evidence for this hypothesis comes from experiments in which the brain of one fly was transplanted to the abdomen of a mutant fly, which was arrhythmic. The recipient became rhythmic, presumably due to rhythmic secretion of hormone from the transplanted brain.

Genetic experiments have also suggested the duplication of clock organization in the two sides of the fly brain.

In the marine mollusk Aplysia, a separate clock located in each of the eyes of the animal has been identified. An eye can be removed and maintained in culture, where it continues to exhibit a circadian rhythm of neural activity. In the closely related mollusk Bulla, rhythmicity in culture can be maintained by as few six cells located at the base of the eye.

In vertebrates, two structures associated with different part of the brain have been implicated as controlling centres of the circadian clock of the animal. In the house sparrow, the circadian rhythm of locomotory activity can be stopped by removal of the pineal gland.

General property of Circadian rhythms

The circadian rhythms that is controlled by an internal undamped oscillator mechanism have following characteristics:

Self-sustainity and persistency A noticeable characteristic of many biological is their endogenous nature. Though biological rhythms evolved in response to a periodic environment, they are driven by mechanisms that are independent pf periodic environ­mental input. A classic example of an endogenous rhythm is the pattern of wheel-running activity in the golden hamster. 1.     Temperature compensation Circadian clocks enable the organ­isms to determine the time of day. Although metabolic reactions doubles with each 10 oC increase in temperature, the period length of circadian rhythms under constant laboratory conditions is the same at many different temperatures. This property is known as temperature compensation and clearly indicates that biological rhythms are controlled by a clock mechanism.2.     Entrainment Circadian clocks must be synchronized, or entrained, to the 24-hour day. Circadian rhythms are sensitive to small amounts of light. For example, only 1 second light per day is enough to entrain the circadian period of locomotor activity in the golden hamster.  


Application of Biological Rhythms


Human Health

Circadian timing of susceptibility to toxins, stress, and therapeutic procedures is often dramatic and must eventually play a major role in practical medicine.


Jet lag


Manic depression


Seasonal affective disorder


Sleep disorder





Gr.    Greek      La.    Latin


ablation [La. ablatio] the removal of tissue, a part of body, or an abnormal growth, usually by cutting.

acrophase [Gr. akros extreme] phase angle of the top of a sine func­tion fitted to raw data of a rhythm.

active element component in a biological system that can generate itself independently oscillations.

active oscillating system a system capable of self-sustained oscillations (endogenous rhythms).

aftereffects generally an effect following its cause after delay, espe­cially a delayed physiological or psychological response to a stimulus. In biological rhythms, it refers to long-term transients after a rhythm is released into constant conditions until it begins the spontaneous free-running period.

amplitude [La. amplus large] difference between maximum or minimum and mean value in periodic oscillations.

biological clock an internal timekeeping mechanism capable of driving or coordinating a circadian rhythm.

Circadian taken from Latin words meaning "around" and "day"

biological rhythm self-sustained cyclic change in a physiological process or behavioural function of an organism that repeats at regular intervals.

circadian rhythm [La. circa, about + La. dies, day] self-sustained biological rhythms that in the natural environment is entrained to a 24-hour period.

Circannual a biological rhythm with a period of about one year

circalunar rhythm [La. luna, moon] self-sustained biological rhythms that in the natural environment is entrained to the period of moon, 28 days.

circannual rhythm [La. circa, about + La. dies, day] Self-sustained biological rhythms that in the natural environment is entrained to a period of the 365.25-day seasonal variation in the invironment.

circa-rhythms A group name for circadian, circatidal, circalunar, and circannual rhythms. These classes of rhythms are able of free- running in constant conditions with a period nearly that of the environmental cycle to which they are normally synchronized. They can be entrained by zeitgebers.


circatidal rhythm [La. circa, about + tide, periodic variation in the surface level of the oceans, caused by gravitational attraction of the sun and moon, with the lunar effect being the more powerful.] Self-sustained biological rhythms that in the natural environment is entrained to a period of the ocean tides, circa 12.4 hours.


culture A population of microorganisms, usually bacteria, grown in a solid or liquid laboratory medium.

chromosome [Gr. chroma color + soma body] one of the threadlike structures in cell nucleus that carry genetic information in the form of genes. It is composed of a long double filament of DNA coiled into a helix together with associated proteins, with the genes arranged in a linear manner along its length.


desynchronization [La. de, from, remove + Gr. sun-, same + Gr. khronos, time] Loss of synchronization between two or more rhythms so that they show independent periods.


dexterity [La. dexter skilful, on the right side] Skill in use of the hands or body.

Diurnal performed in or belonging to the daytime; opposite of nocturnal

DNA (deoxyribonicleic acid) the genetic material of nearly all living organisms, which controls heredity and is located in the cell nucleus. Changes in the DNA cause mutations.

Eclosion emergence of the adult insect from its pupal case

Endogenous self-sustained rhythm generated within an organism

endogenous rhythm [Gr. endon, within + Gr. gennan, to produce] Self-sustained rhythm generated within an organism. That is, the rhythms are persisting even when all known environmental cycles are eliminated. Endogenous refers to that the rhythm is intrinsic to organism and are not derived from environmental variations.


entraining agent Synonymous with zeitgeber. an environmental time cue such as light that has the ability to reset a biological clock


entrainment [La. en-, away + trainer, to drag] Synchronization of an endogenous rhythm by another rhythm (i.e. zeitgeber). During entrainment the frequencies of the two oscillations are the same or integral multiples of each other and they have a constant phase relationship, or phase angle difference.


exogenous rhythm [Gr. exo outside + Gr. gennan to produce] Rhythm generated by the influence of an environmental periodicity on an organism.


external desynchronization [La. externus outward] Loss of synchronization between rhythm and the forcing oscillation (zeitgeber).


forced internal desynchronization When an organism are exposed to one or more zeitgeber cycles which entrain only some of its circa- rhythmic variables, it occurs internal desynchronization.


free-run State of a circa-rhythm in absence of zeitgebers, that is, in constant conditions.


free-running period (t) When a biological clock is not entrained by some forcing oscillation (zeitgeber), it shows its fundamental period that is called free-running period too. Put differently, natural self-sustained rhythm that exists in the absence of all environmental cues. When a human is free running, his/her cycle appears to be slightly longer than 24 hours.


frequency [La. frequens crowded] The number of times a specified phenomenon occurs within a given interval. It is reciprocal of period.

frequency demultiplication If a biological rhythm is entrained by a zeitgeber which its period is an integral fraction of the rhythm’s period, occur frequency demultiplication.


gene [Gr. gennan to produce] the biologic unit of heredity, self-reproducing and located at a definite position on a particular chromosome.

gene clone [Gr. klon young shoot or twig] a group of identical genes produced by techniques of genetic engineering.

Heliotrope one of several species of plants whose flowers or stem buds face east in the morning, follow the movement of the sun during the day, and face west in the evening

Homeotherm organism whose internal body temperature is maintained at a relatively constant temperature through the organism's metabolic activities; its body temperature typically does not change in response to changes in environmental temperature fluctuations; homeotherms were once called "warm-blooded".

Hypothalamus small area of the brain near the top of the brain stem; control site of behaviours such as feeding or drinking, temperature regulation, secretion of hormones through its effect on the pituitary gland.

infradian [La. infra beneath + La. dies day] A biological rhythm occurring in cycles of more than 24 hours.

internal desynchronization Loss of synchronization between two or more rhythms so that they free-run with different periods within the same organism.

invertebrate an animal without a backbone (spinal column).

lesion [La. laesio, laedere to hurt] a zone of tissue with impaired function as a result of damage by disease or wounding.

locomotion [La. locus place + movere to move] movement or the ability to move from one place to another.

locomotor of or pertaining to locomotion; pertaining to or affecting the locomotive apparatus of the body.

locomotory pertaining to locomotion.

mean value The arithmetic mean of all instantaneous values of an oscillating variable within one cycle.

mediator [La. mediat to be in the middle] To mediate means to trans­mit or convey as an intermediary agent or mechanism. In biorhythm’s context, neural or endocrine function which through its oscillations can transmit period and phase information so as to synchronize the rhythms in a target tissue.

Melatonin a hormone secreted by the pineal gland used as a marker of circadian rhythmicity in humans.

mollusk any of a class of animals which have soft bodies without a backbone or limbs and are usually covered with a shell.

mutant an individual in which a mutation has occurred.

mutation [La. mutatio, from mutare to change] a change in the genetic material (DNA) of a cell, or change this causes in a character­istic of individual, which is not caused by normal genetic processes.

oscillator [La. oscillare to swing] Any device which, in the absence of external force, can have a periodic back-and-forth motion.

pacemaker A functional and localizable entity which has self-sus­taining oscillations and synchronizes other rhythms.

passive element A component in biological system which can not generate self-sustained oscillations. A passive element is rhythmic only if forced by another oscillation.

passive oscillating system A system capable only of forced (exogenous rhythms) or free damed oscillation.

period [Gr. periodus circuit : peri around + hodos way] Time for the regular recurrence of a defined phase of the rhythm. It is the time that requires for completion of one cycle.

phase [ Gr. phasis an appearance] Any instantaneous state of oscillation within a period.

phase angle Relative term measuring the relationship between a particular phase in a cycle and some arbitrarily chosen reference point or phase of another rhythm. The whole period is defined as 360 degrees.

phase angle difference Difference between corresponding phase angle in two coupled oscillations. It can be measured in unit of time or angle. For example, if body temperature minimum occurs at 9:15 P.M. and sleep onset at 10:45, then the phase angle differences between these two rhythms is 1.5 hour.

phase control Control of the period and phase relationship of a rhythm by a zeitgeber.

phase relationship Synonymous with phase angle difference.

phase response curve A graphical plot that shows how the amount and direction of a phase shift induced by a single stimulus depends on the time, or phase, at which the stimulus is applied.

phase shift Single displacement of an oscillation along the time axis. It may happen without delay or after several transient cycles. By means of phase shift a biological clock is reset.

photoperiod Duration of light in a light-dark cycle.

plankton [Gr. planktos wandering] A collective name for the small free-floating organisms, vegetable and animal, which live in water especially the sea.

Poikilotherm organism whose internal body temperature varies with and remains close to environmental temperature; poikilotherms were once called "cold-blooded"

Pulsatile rhythmic regular movement

range of entrainment Range of periods within which a self-sustaining oscillation can be entrained by a zeitgeber.

range of oscillation difference between maximum and minimum value, independent of shape of oscillation.

relative coordination when an organism is exposed to a too weak zeitgeber cycle, its rhythm does not entrain, but modulations in the period occur, which are called relative coordination.

secondary oscillator a kind of oscillator within an organism can generate oscillations. But secondary oscillator has less stability and persistence than a pacemaker. It cannot be entrained directly by zeitgebers, and may not necessarily synchronize other oscillators.

sleep debt deficit in normal sleep time.

Tau this term refers to an organism's period length

Temporal of or relating to time.

transducer [La. : trans, across + ducere, to lead] Generally, a sub­stance or device, as a piezoelectric crystal or a photoelectric cell that converts input energy of one form into output energy of another. In biorhythms context, component in a biological system, which detects environmental zeitgeber, cycles and converts the temporal informa­tion into a form (typically neural or endocrine rhythms), which can synchronize biological oscillators.

transient [La. : trans, over + ire, to go] Generally, lasting only a short time. In biological rhythmicity, temporary oscillatory state between two steady states.

transient internal desynchronization Internal desynchronization occurring between circa-rhythms over only a few cycles. Typically this may happen after a phase shift of a zeitgeber when some rhythms resynchronize more rapidly than others.

ultradian Biological rhythms with a period much shorter than 24 hours.

zeitgeber [German: Zeit, time + geber, giver] This term introduced by J. Aschoff and taken from German words meaning "time giver". Any forcing environmental oscillation, or any external rhythm, which entrains, or synchronises, an endogenous biological rhythm. In the biorhythms, the light-dark cycle is the most powerful zeitgeber, or entraining agent.



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Aschoff, J. (ed.) (1982), Vertebrate Circadian Systems: Structure and Physiology. New York: Springer Verlag.

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Kupfer, D. J. (1988): Biological Rhythms and Mental Disorders. New York: Oxford University Press.

Mendlewicz, J. (1983): Biological Rhythms and Behaviour. Harvard University Press.

Moore-Ede, M. C., Sulzman, F. M., and Fuller, C. A. (1982): The Clocks That Time Us. Basel: Karger.

Moore-Ede, M. C. (1992): Electromagnetic Fields and Circadian Rhythmicity. Boston: Birkhäuser.

Pierpaoli, W. (1994): The Aging Clock, New York Academy of Science, New York.

Saunders, D. S. (1982): Insect Clocks, Oxford: Pergamon.

Schulz, H. (1985): Ultradian Rhythm in Physiology and Behaviour. Berlin: Springer Verlag.

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Wever, R. A (1979): The Circadian System of Man, Results of Experiments under Temporal Isolation, Springer Verlag, New York.

Winfree, A. T. (1978): The Timing of Biological Clocks, Scientific American Books, New York.

Waterhouse, J. M. (19xx): Circadian clocks and their adjustment,.



A. Human’s Rhythms

Humans display a wide spectrum of frequencies in their rhythmic processes.

Frequency (cycles/s)                            Periodicity (T=1/f s)

A1. Ultradian Rhythms

Endogenous rhythms that occur in less than 1 day ranges are called ultradians rhythms. Ultradian rhythms occur in a wide range of fre­quencies,

Alternation between REM and non-REM stages of human sleep T= 90x60 to 100x60 s

Breathing (Respiratory)

EEG (Electroencephalogram)

Heart beat (Cardiac)


Neural activity in the suprachiasmatic nucleus

Sleep-stage (Rate of progression through different levels of sleep)

A2. Circadian Rhythms

Blood pressure

Body temperature

Cell division

Cortisol secretion from the human adrenal gland

Drug sensitivity/ metabolism

Feeding / Metabolism

Hormone level

Locomotory activity

Manual dexterity

Mental performance/ alertness

Pain sensitivity

Physical performance

Sleep and wakefulness

Urinary electrolyte

Urine excretion


A3. Infradian Rhythms

Endogenous rhythms that occur in greater than 1-day ranges are called infradians rhythms.


Hibernation in ground squirrels

Menstrual cycles of women

Migration patterns of garden warblers

The plant seed’s annual capacity to germinate



B. Environmental Cues

Activities of other organisms

Cycles of eating and fasting (EF)


Drugs (lithium, clorgyline, benzodiazepines, triazolam, diazepam, etc.)

Light-dark cycle

Light lux

Neurotransmitter-related substances (muscimol, GABA, carbachol, neuropeptide Y, etc)

Physical activity

Social factors

Temperature oC



D. Proposals to Experiment

1) How does effect the shift work on the shift worker at Volvo?

2) How does effect the shift work and night work on the personals at Sahlgren Hospital University?

3) What are the effects of flying to east or west over the pilots in Gothenburg Airplane?

4) Can show any the correlation between women menstruation and lunar rhythms?












Earth’s rotation







Model’s Variables

Body/ Rectal Temperature        oC

Light     lux

Melatonin (MEL) Level            ?

Time     s

Wakefulness/ Sleep

Urinary Cortisol Excretion                  mg/h