One of the features of life is the periodic, regular occurrence of the same processes. Recurring external conditions make an imprint on all living organisms enabling them to fit into schedules of behaviour. First to consider is the relationship between rhythms and the environment. A rhythm is a recurring, wavelike character with a maximum and minimum states appearing at identical intervals of time. The distance between the identical positions of two waves, whether maxima or minima, constitutes the period of the rhythm. A cycle has two phases, a rise and fall, a strengthening and a weakening. In neurophysiology this is exhibited by the complimentary and contradictory phenomena of excitation and inhibition in nervous transmission. An example of a natural cycle is the diurnal cycle of illumination, of light and dark. The amplitude is the range of fluctuation, the degree of deviation, from some mean value. For example, with diurnal animals respiration is strongest in daytime, weakest at night. The daytime-night-time deviation of respiration from the mean 24 hour level is therefore known as the amplitude of diurnal rhythm.
The great diversity of biological rhythms is amenable to systematisation. Some rhythms coincide in duration to some geophysical cycles. These are ecological rhythms. Included in this classification are the diurnal, tidal, lunar, and seasonal variations in life processes. As well as these, physiological or functional rhythms ensure the constant requirements of life. Ordinarily these are short cycles and are the result of variations in the state of molecules. Biological rhythms differ according to their dependence upon the environment. Exogenous rhythms depend upon external stimuli, such as variations in the configuration of atoms and molecules. Life chemicals do not just exist in space, they do not only possess spatial existence, they also exist in time. Examples are to be found in pigment molecules, muscle contractions, tropisms in plants, and animal taxes.
An endogenous rest can be seen as sleep, cessation of growth, or development, whilst the organism restores its capabilities. An endogenous rhythm with its durations in cycle, deviates from 24 hours. This deviation takes place within the framework of an environment that is relatively constant. In plants this deviation spans 23 to 28 hours. In animals this fluctuation is between 23 and 25 hours. These are known as circadian rhythms. This alternation of phases in endogenous rhythms can be described as an inner automatism. The influence of the environment and innate rhythms are synchronised to corresponding external cycles. These external conditions can be seen as unitary signals, synchronisers, time cues or time givers. Synchronisers as unitary signals function whether they are gradually changing conditions or sudden changes. Some affect organisms regardless. Such synchronisers are day and night, or temperature variations. Light, however, is a universal synchroniser.
The basis of life is chemical reactions, the rate of which depends upon temperature, there being an optimum point of chemical activity. Vant Hoff and Arrhenius put forward the concept that the rate of a process is affected either plus or minus, by a factor of 2 or 3, when temperature fluctuates by 10 degrees C. Investigations into processes measuring astronomical time, such as photosynthesis, bioluminescence, or locomotion, show that in these cases temperature coefficients of cycle duration are close to unity. Hence the rate of rhythm process is practically independent of temperature. Underlying circadian rhythms are not just chemical processes, but other processes too, and thus are only slightly dependent upon temperature changes within 10 t0 30 degrees C.
Every 24 hours there are changes in the illumination, temperature, and the relative humidity of the environment. Certain invertebrates and plants are adapted to this rhythm. The amplitude of the diurnal rhythm is affected by barometric pressure, ionising radiation, and the electric potential between earth and the atmosphere. Hence noises, colours, feeding times, and sexual stimuli, can act as synchronisers of the biological rhythms. However, these cues are weaker than the rhythms of light and dark.
With regard to time and reflex activity we come to rhythms in higher organisms. Higher organisms, in the sense of possessing developed nervous systems, of varying integrative capabilities. Every nerve cell is continually in a state of dual excitation and inhibition. Excitation can be seen as activity, and inhibition as relative rest. A cell in a resting phase still possesses intracellular activity, it does not cease to function, it is the timing, the rate, that alters in its basic reactions. The activity of the brain in humans, for example, is based upon not only unconditioned or innate reflexes, but also upon conditioned or acquired reflexes. Unconditioned responses can only partially adapt an organism to life in a certain environment. Conditioned reflexes complete the comprehensive relationship with the environment.
The question of the mechanism of a conditioned time reflex is not entirely clear, the problem being the role of physiological time acting as a conditioned stimulus. The body possesses cyclic phenomena, and every state of an organ is known by the cerebral cortex. Hence the organism possesses a good basis for distinguishing instants of time. We can see, therefore, that all time measured activities of ab organism are based on the rates of development of various physical and chemical processes going on in, and between’ cells and tissues.
To consider the basic premise, without making reductionist or nothing-but mistakes, we can infer that DNA, RNA, and protein synthesis is time controlled. DNA can ‘tell the time’ – not a human concept of time, but a rate of time, adapted through evolution to achieve a unity between organism and environment. The essence of life chemistry is its recurrence – life chemistry enters into time because it replicates, it repeats, and thus DNA, unlike non-recurring inorganic matter, has not only a present but a past and a future. This needs further clarification because DNA replication infers a level of determinism, of inevitability, a relationship of cause and effect, where divine intervention is an irrelevant concept, but where biochemistry and the philosophy of science become entwined.
With regard to the development of diurnal rhythms in humans, physiological activities are typified by the rhythms that persist under constant illumination. These are body temperature, respiration, contractions of the heart, blood pressure, eosinophil level, serum iron levels, and the elimination of electrolytes. All these processes possess a circadian rhythm. An example of an animal adaptation to human rhythms is the feeding time of the mosquito – at night – when in humans the peripheral circulation is at its highest.
A relative role exists between heredity and the environment in the development of diurnal rhythms. The diurnal fluctuations of various functions are established at different times. In all physiological processes adults characteristically show single phase diurnal rhythms. However in infants, especially the neonate, there are several phases – examples of which can be seen n patterns of sleeping and feeding. Single phase diurnal rhythms appear largely irrespective of the environment. Towards old age the body again becomes poly-phasic. Here we can see the role of metabolic processes in the onset of ageing – and the possibility that ageing and death are as inbuilt as the processes of growth, development and maturation. Therefore dying is just as much a part of life as being born. What is inherited are not the rhythms but their potential for development. In the process of development it is only the innate propensity for circadian rhythms that is manifested. Humans are potentially able to adapt to periodicity to non-terrestrial environments.
To retain diurnal rhythmicity of physiological functions it is important that periods of light and wakefulness coincide. Humans emerged with an inherited central pacemaker, a master clock, that is primarily regulated by lighting conditions. The principal signal for sleep is the darkness, which entails a stop in the flow of exciting signals to the retina of the eye. Nocturnal animals are adapted to different wavelengths of light, hence their wakefulness is photically stimulated by wavelengths unperceived by diurnal organisms.
Circadian rhythms display general regularities. All organisms thus measure the time of day and night. The basis for this ability is an innate, almost diurnal and circadian rhythmicity of metabolic processes. There exists a direct relationship between the level of the organisation of matter, the life times of appropriate structures, and the durations of cycles. All characters and properties arise on the basis of a random and undirected hereditary variability – and these are reinforced and established by natural selection. Only those organisms survive which are better adapted to the conditions of life. The circadian periodicity of the metabolic processes has proven to be an important adaptation to environmental diurnal variations. With regard to animal behaviour rhythmicity appears to be of primary importance, and the concept of time and rhythm can be of use in the explanation of displacement activities, learning, mating, feeding, and of the complexities of motivation and drive.