Circadian systems: different levels of complexity.

After approximately 50 years of circadian research, especially in selected circadian model systems (Drosophila, Neurospora, Gonyaulax and, more recently, cyanobacteria and mammals), we appreciate the enormous complexity of the circadian programme in organisms and cells, as well as in physiological and molecular circuits. Many of our insights into this complexity stem from 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 oscillators, multiple input pathways and feedback loops that contribute to specific circadian qualities but not necessarily to the generation of circadian rhythmicity. For a full appreciation of the circadian programme, the results 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. This review summarizes some of the complexities found at the level of organisms, cells and molecules, and highlights similar strategies that apparently solve similar problems at the different levels of the circadian system.