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By evolutionary adaptation to the perpetual day-night changes in envi ronmental conditions, eukaryotic organisms have acquired an endogen ous programme. This mechanism exhibits the characteristics of a self sustaining oscillation the period of which approximates that of the earth’s rotation. For animals such a property was first clearly demon strated by Maynard S. Johnson (1939) who recorded, in constant conditions, free-running activity rhythms of white-footed mice (Peromyscus /eucopus). Johnson concluded from his observations that 'this animal has an exceptionally substantial and durable self-winding and self-regulating clock, the mechanism of which remains to be worked out'. Twenty years later, the formal properties of this 'circadian' clock and its use by organisms as a time-keeping device were summa rized at the Cold Spring Harbor Symposium in 1960 (Chovnick 1961). During the following two decades, investigations have turned towards an analysis of the physiological mechanisms involved in and the search for a central masterclock. These efforts led to the discovery that the pineal organ of submammalian vertebrates and the suprachiasmatic nuclei of birds and mammals are major candidates for a role as central circadian pacemakers. At the same time the neural pathways through which these structures are coupled to the light-dark cycle were identi fied. Furthermore, it was established that the pineal gland and the suprachiasmatic nuclei are closely related structures that integrate the functions of circadian timekeeping and photoperiodic time measure ment.
