In this article we will discuss about:- 1. Meaning of Circadian Rhythm 2. Examples of Circadian Rhythm 3. Diagnostic Features 4. Mechanism.
Meaning of Circadian Rhythm:
Circadian rhythm among all other rhythms have been widely studied. The term originated from two Latin words, circa meaning about and dian meaning a day. Circadian rhythm can be defined as an approximately 24 hour physiological pattern that exhibits daily changes in activities. Circadian rhythms are easy to study as they recur within 24 hours in the activity of an individual.
The behavioural response of organisms to environmental rhythms of light and darkness is called photoperiodism, which includes period of illumination (photo phase) followed by a period of darkness (scatophase). Such daily cycles are termed as photoperiod. Animals are not active continuously throughout the 24 hour period.
Majority of them are most active during the daylight hours (diurnal animals). While others are active at night (nocturnal animals). A few animals such as rabbits, sandflies etc. are crepuscular animals who are active during the twilight hours of the day, in early morning (dawn) or early evening (dusk).
Examples of Circadian Rhythm:
A. In Invertebrates:
Circadian rhythm in invertebrates occurs in a number of behavioural patterns ranging from vertical migrations by marine plankton, periodicity or pupal eclosion in dipterians to the dance ‘language’ of foraging bees.
A few common examples are:
1. In a lake or ocean, phytoplankton photosynthesizes during the daylight hours, occupying the upper region. Zooplankton remain well below the surface at midday when the sunlight is most intense. As darkness approaches, these organisms move upward to feed on the phytoplankton. They sink back to deeper waters after sunrise.
2. Cockroaches are nocturnal and most of their locomotory and feeding activities occur during the hours of darkness. When these animals were placed under experimental situations in which the obvious diurnal factors of light and temperature were removed, it was observed that the rhythmic activity persisted but with a period, which departed significantly from that of the solar day. This natural endogenous circadian clock which is characteristic of an individual cockroach is not calibrated to exact 24 hours condition, but may vary between 23 and 25 hours.
3. Honeybees have great foraging powers. Some flowers blossom once in a day at a specific time. Honeybees reach these flowers at the same time with the help of ‘alarm’ of its own biological clock.
4. Some intertidal crabs exhibit cyclic variation in their shell colour. They are lighter during the daytime and gets darker as night falls. This prevents their detection from predators.
5. Parasitic animals also show circadian periodicity. The microfilarae of African eye worm (Loa loa) appear in the peripheral circulation of the host (man) in daytime only, going deeper during night. Wuchereria bancrofti (filaria worm) show nocturnal periodicity.
The microfilarae of this endoparasite live by daytime chiefly in large deep-seated blood vessels. But at night they come to lie in the small superficial vessels in the skin. Enterobius vermicularis (pinworm) live in the colon (small intestine) of man at daytime. The gravid females of this worm migrate to perianal region at late night, to deposit eggs in the skinfolds around the anus.
B. In vertebrates:
A few examples of vertebrates exhibiting circadian rhythm are given:
1. To study the circadian periodicity in mouse, a cage outfitted with a device that recorded all the movements of the mouse was taken. When all the records were summarised graphically with appropriate units for the time axis, it revealed a cycle of activity which often reached the peak just after sunset. Very little activity was observed in the middle of the day. Further studies have revealed that hormones were secreted by the hypophysis according to diurnal rhythm.
2. In another experiment Patra (1984), while studying the circadian rhythm of oxygen consumption in three air-breathing fishes (Ambus, Clarias and Channa), observed a clear rhythm in their metabolism. These fishes were most active during the morning hours, when they moved about in search of food. From afternoon onwards they became inactive or remained submerged in water. This rhythmicity is closely related with the diurnal fluctuation of dissolved oxygen.
3. Circadian rhythms may change with the change of place and may also change with age. For example, young woodchucks remain active during the evening only, while their adults remain active almost throughout the day. Similarly, young badgers remain active during the early hours to the middle of the day. As they grow older, their activity time is shifted from evening to night.
4. Two species occupying the same habitat differ in their daily activity rhythms to reduce competition. For example, fly catching birds and bats never come in direct competition for the same food as the birds are diurnal and bats nocturnal.
5. A few birds and smaller mammals, in spite of being homeothermal, show diurnal variations in their body temperature. This is correlated with their changed metabolic activity during day and night. For example, black capped chickadees can decrease their body temperature to as much as 7°C during the night, thus reducing their energy demand by 60%, as the temperature falls in winter.
6. Rhythmicity is also observed in the physiological and biochemical reactions within the body of living organisms. Vijay Lakshmi (1984), while experimenting on albino rats, studied synchronized shifting of liver glycogen and ATPase activity under varied light regime. She maintained rats under 12 hours’ light followed by 12 hours’ darkness, and then in reverse order.
She also maintained a set of rats under constant light and another set under constant darkness. In the last two situations, rhythmic trend in glycogen, glucose and energy linked enzyme ATPase showed entrainment. Thus, her studies indicated the interdependence of the metabolic and related enzyme activity under varied photoperiodism.
Human circadian rhythm:
Human body also goes through regular biological cycles every day, many of these reach a peak at some regular point during the day or night. G. G. Luce (1973) has suggested that human blood-sugar level, pulse-rate and blood- pressure reach peaks at 5 to 6 p.m. Humans show a periodicity of 24 hours in case of various physiological activities such as hormone levels in blood, blood pressure, EEG, ECG and other activities such as rest cycle, body temperature etc.
The secretion of various hormones from different endocrine glands shows a rhythmic pattern. The most important gland exhibiting this rhythmic pattern is pineal gland. Serotonin hormone, the secretion of this gland, is highest in noon and lowest at midnight, whereas melatonin hormone is highest at night and it stops in the early hours of morning (increased secretion of melatonin hormone makes a person lazy).
Performance of different mental activities such as calculation task, vigilance task, sensory and motor performance, correct signal detection etc., has been found to be affected by the time of day. They are high in the afternoon and early evening and low at night and early morning.
On the other hand, performance on digit span was best in the morning. Thinking also seems to have rhythmic pattern, which slows down at night and increases in the morning. Sensory abilities such as hearing, vision taste and smell are best in late afternoon.
On the basis of certain rhythms, such as compulsory or elective activity, financial or prestigious incentives, social condition and personality etc., two types of persons have been identified – morning types and evening types.
Morning types are at their best in the morning while evening types in the evening. This has a practical implication in industries, such as, morning types should be given morning shifts and evening types evening shifts, which would likely increase their performance.
The above types are also related to personality. Introverts are found to be morning types, while extroverts are evening types. Introverts have higher temperature in the morning and are more aroused during this time. Extroverts, on the other hand, are more aroused during the evening as they have higher temperature than introverts during this time.
On the basis of some practical work conducted on man, Bernard Gittelson was able to distinguish three types of biorhythms:
(i) Physical rhythms, which makes individual feel very active or very sluggish;
(ii) Emotional rhythms, which controls the emotional cycle of an individual; and
(iii) Intellectual rhythms, which is responsible for intellectual alertness and dullness.
Gittelson further put forward that on the basis of the date of birth of an individual, three different charts can be plotted of these three rhythms. If individuals worked according to this chart, he would be benefited to a greater extent.
When all these cycles are at their peak then maximum performance would be expected from an individual. Thus, circadian rhythm affects our day to day performance and, through proper knowledge of these patterns, an individual can achieve his best.
Diagnostic Features of Circadian Rhythms:
1. Circadian rhythm must also persist in laboratory under conditions of continuous light or total darkness and under constant temperature.
2. Circadian rhythm must be able to express itself in light-dark conditions as free running rhythm, showing periods close to – but not equal to – 24 hours.
3. Its free running period must be able to compensate for changes in the surrounding temperature.
4. A circadian rhythm is known to be entrained (it is a process by which an endogenous rhythm is linked with an external geophysical rhythm) by the light-dark and temperature cycles.
5. A circadian rhythm must be able to shift and reset its phase in response to a change in light and dark conditions, and to temperature or chemical disturbance.
Mechanism of Circadian Rhythm:
Three components are believed to be operative for circadian rhythm.
(a) An Oscillator:
As circadian rhythm is also observed in acellular organisms, it is probably that the oscillator of biological clock resides in the cell. Three theories have been put forward for the precise identification of the clock in the cell:
(i) The plasma membrane:
As the permeability of plasma membrane changes periodically at different times in 24 hours, Giese (1989) is of the opinion that the physiology is changed in circadian way.
(ii) The nucleus:
As rhythm is absent in prokaryotes, it is supposed that the clock might be in the nucleus and acts by changing levels of macromolecular synthesis.
(iii) Geophysical variables:
Some geophysical variables are believed to be responsible for the changed function of a cell membrane or nucleus or both.
(b) A Receptor:
Certain receptors are responsible for the input to the oscillator. These may be in the form of photo-receptors present either in the eyes or pineal gland or in the brain itself.
(c) A Coupling Device:
It is believed that the device that couples the receptor to the oscillator is chemical in nature. Experiments conducted by Truman and Riddiford (1990) with two species of silkworm has revealed that the eclosion (emergence) of moths is controlled by a hormone (chemical) in the brain which can be used to induce eclosion in another species of silkworm. Thus, the coupling device is not species specific.