Exogenous and Endogenous Components in Circadian Rhythms

The main topic of the symposium "Biological Clocks" says nothing about the types of clocks we will be concerned with. It is an open question whether the clocks run continuously or stop after one revolution and have to be started anew. Also the term "clock" does not imply that one revolution is finished in 24 hours. Watches are instruments to measure time. Organisms have to measure time-spans of quite different lengths and for different purposes. Each measurement of speed, for instance, needs timing and mostly within the limits of milliseconds. One can expect, therefore, that organisms possess several clocks with perhaps extremely different periods 1. There is no need for these clocks to run continuously; for some purposes it would be sufficient if the clock were started always at the beginning of timing (principle of a sandglass, stop watch). "Biological clocks" therefore is a more general concept than what...