• Results of biological and psychosocial studies of depression completed in the last decade have stimulated the need for new hypotheses that synthesize these findings in a unified etiologic theory. The importance of disruption of biological rhythms on the one hand, and psychosocial losses on the other, in the causation of depressive episodes suggest one possible unifying hypothesis. The concept of loss of "social zeitgebers," ie, persons, social demands, or tasks that set the biological clock, may provide the link between biological and psychosocial theories of etiology. We suggest that a disruption of social rhythms, which may result in instability in biological rhythms, could be responsible for triggering the onset of a major depressive episode in vulnerable individuals.

https://jamanetwork.com/journals/psych/articlepdf/494403/archpsyc_45_10_012.pdf

Social Zeitgebers and Biological Rhythms: A Unified Approach to Understanding the Etiology of Depression

As in other vertebrates, the human circadian system is characterized by a distinct temporal order of its components. This order is maintained by the coupling forces between various oscillators as well as by the entraining signals from the Zeitgebers (see Chapter 12). There is hardly a tissue or function that has not been shown to have some 24-hr variation. As an example, Figure 1 presents results of an experiment in which six subjects were held in groups of two on the same rigorous schedule. Although there were considerable interindi-vidual differences and also day-to-day variations, patterns like those shown in Figure 1 are satisfactorily reproducible. This reproducibility is again illustrated in Figure 2, which summarizes data on plasma Cortisol collected in six laboratories. To account for possible phase-controlling effects of sleep (see below), the curves are normalized with regard to the various sleep times of the subjects. Despite the fact that Cortisol, like many other hormones, is secreted in a highly variable sequence of episodes (see Chapter 12, Figure 1), a clear circadian pattern emerges in each curve, averaged from the data on n subjects, and there is perfect correspondence in phase and relative amplitude between the curves. It is noteworthy that two submaxima appear in all curves at about the same circadian phases. Presumably, they are not consequences of the usual meal timing but may represent a rhythm component interposed between the circadian domain and the episodes (cf. the discussion in Aschoff, 1979).

The Circadian System of Man

We describe a new syndrome called "delayed sleep phase insomnia." Thirty of 450 patients seen for a primary insomniac complaint had the following characteristics: (1) chronic inability to fall asleep at a desired clock time; (2) when not on a strict schedule, the patients have a normal sleep pattern and after a sleep of normal length awaken spontaneously and feel refreshed; and (3) a long history of unsuccessful attempts to treat the problem. These patients were younger than the general insomniac population and as a group did not have a specific psychiatric disorder. Six patients' histories are described in detail, including the successful nonpharmacological chronotherapy regimen (resetting the patients' biological clock by progressive phase delay). Delayed sleep phase insomnia is proposed to be a disorder of the circadian sleep-wake rhythm in which the "advance" portion of the phase response curve is small.

Delayed Sleep Phase Syndrome: A Chronobiological Disorder With Sleep-Onset Insomnia

In migratory birds, endogenous daily (circadian) and annual (circannual) rhythms serve as biological clocks that provide the major basis for their temporal orientation. Circannual rhythms are responsible for the initiation of migration both in autumn and spring. This function of timing migrations is particularly important for birds that spend the winter close to the equator where the environment is too constant or irregular to provide accurate timing cues. In addition, circannual rhythms produce programmes that determine both the temporal and the spatial course of migration. In Sylvia warblers, the time programmes controlling autumn migration are organized in a species- or population-specific manner. It has been proposed that, in first-year migrants, the time programme for autumn migration plays a major role in determining migratory distance, thus providing the vector component in a mechanism of vector navigation. It is not yet clear, however, whether this programme does indeed determine migratory distance or whether it only provides the temporal framework within which other factors determine how far a bird flies. Evidence against the first alternative comes from findings indicating that migratory activity can be drastically modified by a constellation of rather specific, but highly relevant, factors and that the resulting changes in migratory activity are not compensated by subsequent increases or decreases of migratory activity. In normally day-active but nocturnally migrating birds, circannual signals cause alterations in the circadian system leading to the development of nocturnal activity. Although the nature of these signals is unknown, there is evidence that changes in the diurnal pattern of melatonin secretion by the pineal gland are associated with, and possibly causally involved in, the waxing and waning of nocturnal activity. These changes in the melatonin pattern presumably also affect general synchronization properties of the circadian system to Zeitgebers in such a way that circadian rhythms adjust faster to new conditions after long transmeridian flights.

Circadian and circannual programmes in avian migration

The article reviews most of the data available on the period ẗ of freerunning circadian rhythms, measured in constant conditions. Emphasis is placed on the effects of light intensity and ambient temperature on ẗ, with references to influences of other external as well as internal factors. In the introduction, examples are given of spontaneous and induced variations in ẗ and its dependence on the experimental history. The discussion concentrates' largely on results obtained from arthropods and vertebrates.

Circadian rhythms: influences of internal and external factors on the period measured in constant conditions.

To describe anabrupt shift of the Zeitgeber sufficiently, it is necessary to indicate whether the phase has been advanced or delayed, for how many hours it has been shifted, and what part of the Zeitgeber has been lengthened or shortened respectively. The entrained circadian system usually follows a shift of the Zeitgeber in the same direction and by an equal amount. However, there can be exceptions to both of these rules: a) an instantaneous 12-h shift of a (symmertric) Zeitgeber always represents a delya (= doubling of one-half of its period). In contrast to this, the gradual re-entrainment of the circadian system can be accompalished by either advances or by delays (Fig. 2), it can occur in both dirctions simultaneously by splitting the rhythmic variable (Fig. 1),or it can represent an apparent advance not distinghushable from a delay ( Fig. 6); b) after complietion of re-entraiment, the phase-angle difference psi between organism and Zeitgeber an differ systematically from the pre-shift psi-value...

R Wever

Papers

Re-entrainment of circadian rhythms after phase-shifts of the Zeitgeber.