The biological rhythms, detectable at all levels of organization, constitute a temporal structure in all animal species. These rhythms concern many biological parameters and have clinical implications, mainly in psychiatry.
Human rhythms are determined by endogenous pacemakers, which are located in the hypothalamus. The hypothalamus is in interrelation with other elements of complex human biology such as the endocrine system, which is affected, via the cortex cerebri, by environmental factors such as light, darkness, seasons, noise, food, and stress. Thus, endogenous pacemakers adapt their impulses to other environmental rhythms. These complex interferences regulate our biological clocks. A dysfunction of one factor may induce a rhythm modification, which alters another rhythm, and so on, and may result in a clinical disorder, often a psychiatric illness. In this manner, our living patterns are controlled by the interrelation between endogenous pacemakers and exogenous rhythms.
The biological rhythms of different functions become apparent at different times after birth. In the infant, the development of rhythmicity must represent a combination of the genetic potential of the maturation process in the brain and of the varying influences of environment. The alternation of light and darkness is perhaps the most obvious of external rhythms, but similar alternations of noise and silence and the attention that the infant receives from adults may also be of importance. In the adult, the biological rhythms are represented by the periodic regular cyclic variations of the biological processes, describing a sinusoidal function with individual characteristics of periodicity and amplitude.
The human rhythms are represented mainly by circa-dian and circannual rhythms, characterized, respectively, by a period of 21 to 27 hours and a longer period of more than 27 hours, such as a month or season. The human cir-cadian system is composed of at least two oscillators, which are self-sustained and coupled to each other. One of these oscillators is strong and controls body temperature, REM-sleep propensity, and cortisol secretion; the other is weak and controls the sleep-wake cycle and sleep related neuroendocrine activity. These oscillator systems may be affected by many factors such as organic diseases, drugs, and environmental factors, which may lead to psychological disorders.
Studies of seasonal patterns of incidence of psychiatric disorders have highlighted the role of seasonally regulated environmental factors on internal biological processes. Since ancient times the relationship between seasons and mood has been noted, and numerous investigations have indicated a seasonal variation in the incidence of affective illness.
Depression has been described as most common in spring and autumn, and the influence of climatological factors (mainly photoperiod) on seasonal affective disorders (SADs) have been shown. One study reported cases of SAD with summer depression and winter hypomania, and Lemoine described summer SAD (or SAD reverse) in which the temperature factor was more implicated than the daylight factor. One biological explanation has been a seasonal variation in human brain serotonin concentrations, which has been implicated in the biochemistry of affective disorders.
But if seasonal rhythms influence depressive illness, a dysregulation of circadian rhythms was found as well. There is evidence that the sleep and neuroendocrine dysfunctions observed in depressive patients are correlated with a phase advance of the circadian strong oscillator with respect to the weak oscillator. Clinical studies suggest that antidepressants can slow or delay circadian rhythms. Other therapies modifying biological rhythms may improve depressive mood. Sleep deprivation, for example, has been found to lead to rapid improvement of depressive symptomatology, and reports have shown that artificial lengthen ing of the photoperiod (phototherapy) may have therapeutic effects in depressive illness. The biological parameter implicated in the mechanism of action of this therapy is melatonin, for which rhythm appears to be an endocrine code of the environmental light-dark cycle conveying photic information that is used by an organism for both circadian and seasonal temporal organization.
Some authors have suggested a relationship between the season of one's birth and the occurrence of affective disorders. Season of birth/conception has also been examined as a possible factor in the depression of women who have given birth. However, although a significant seasonal variation in the occurrence of postnatal depression has been found with the largest peak occurring in autumn, there are discrepancies in the data concerning the influence of the season of conception on the frequency of postpartum mental illness.
The seasonal variation in suicides has been studied in several countries. Suicides were found to be most frequent in spring and summer in Finland and in May and September in France. Seasonal variations of other psychiatric illnesses have been less studied, although a possible link between season of birth and schizophrenia (winter and spring peaks) has been described. Biological reasons may exist, as dopamine has been implicated in the biochemistry of schizophrenia, and there is a seasonal variation in human brain dopamine concentrations.
Human performance efficiency also has circadian rhythms in healthy individuals. One study determined that a simple manual dexterity task is almost entirely under the control of the temperature rhythm oscillator, whereas a more complex cognitive task demonstrates a periodicity that appears to be influenced by those oscillators controlling temperature and the sleep/wake cycle.
Even for human sexuality seasonal variations exist, as they do in other mammals, with a peak in autumn, probably linked to the seasonal variation of testosterone activity.
Abetter knowledge of all these rhythm interferences and their clinical implications brings to mind the possibility that by modifying these influences we may be able to alleviate the patient's symptoms. New approaches to the treatment of all these disorders involve direct manipulation of the biological rhythms.
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