ABSTRACT
This chapter provides an overview of the mechanisms that regulate, generate and coordinate circadian rhythms. By the mid-twentieth century the stage was set for a more formal description of circadian rhythmicity. The clock of ectotherms would be useless if it were not. Circadian clocks do not obey the Q10 rule as a rise of 10 degrees centigrade does not double the period of the oscillation. The existence of circadian rhythms and their basic properties of persistence under constant conditions, temperature compensation and entrainment were established by the 1970s, and interest began to shift to more mechanistic questions. The circadian pacemaker located within the suprachiasmatic nuclei coordinates a circadian rhythm of wakefulness throughout the day and sleep during the night. Core body temperature and/or plasma melatonin also appear to be important in the consolidation of the sleep of humans and other diurnal mammals.
