The Circadian System of Man

Abstract

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).