ABSTRACT
An internal or endogenous clock (or set of clocks) is proposed as some sort of biological mechanism that sets a pace. Such a biological clock would be innate - but then so are many Zeitgebers: our response to lightness is automatic and innate. Non-human animal studies also support the existence of an endogenous clock. Rae Silver and her team have conducted research on endogenous clocks in pigeons and in hamsters. Evidence for biological clocks comes from even the simplest organisms. For instance, single-celled algae from the intertidal zone burrow up to the sand surface as the tide ebbs and go back under the sand again as the tide flows in, so that they can photosynthesise without being washed away. Evolutionary biopsychology is now suggesting that these biological clocks started as single, light-sensitive molecules in primitive bacteria and have evolved into current, complex structures.
