Biorhythms on cellular and organismic level (introduction).

The regular change of day and night, of light and darkness during millions of years has strongly affected the development of life on earth. Many organisms adapted themselves to this environmental condition and, finally, evolved an endogenous timer which usually is in phase with the earth's rotation and causes many functions to perform one oscillation per day. Such circadian rhythms (derived from circa dies i.e. about 1 day) were found in almost all classes of plants and animals, and even in protozoans. They persist in a constant environment and, therefore, are independent of any known external trigger signals. Since even unicells perform circadian rhythms which are similar to those observed in highly developed multicellular organisms many scientists favor the existence of a basic mechanism common to all kinds of biological clocks that is located somewhere in the single cell and probably comprises many different biochemical reactions. One purpose of this topical meeting was to discuss how organisms respond to the absence of gravity and terrestrial zeitgeber and how they may react to the imposing of hypergravity fields. Another aim was to develop model-mechanisms appropriate to describe these responses.