Circadian systems: different levels of complexity

After approximately 50 years of circadian research, especially in selected circadian model systems (Drosophila, Neurospora. Gonyaulax and, more recent ly, cyanobacteria and mammals), we appreciate the enormous complexity of th e circadian programme ill organisms and cells, as well as in physiological and molecular circuits. Many of our insights into this complexity stein fro m experimental reductionism that goes as far as testing the interaction of molecular clock components in heterologous systems or in vitro. The results of this enormous endeavour show circadian systems that involve several osc illators, multiple input pathways and feedback loops that contribute to spe cific circadian qualities but not necessarily to the generation of circadia n rhythmicity. For a full appreciation of the circadian programme, the resu lts from different levels of the system eventually have to be put into the context of the organism as a whole and its specific temporal environment. T his review summarizes some of the complexities found at the level of organi sms, cells and molecules, and highlights similar strategies that apparently solve similar problems at the different levels of the circadian system.