Abstract
         
         Investigations on circadian rhythms have markedly advanced our understanding of health
            and disease with the advent of high-throughput technologies like microarrays and epigenetic
            profiling. They elucidated the multi-level behaviour of interactive oscillations from
            molecules to neuronal networks and eventually to processes of learning and memory
            in an impressive manner. The small-world topology of synchronized firing through neuron-neuron
            and neuron-glia gap junctions is discussed as a mathematical approach to these intensively
            studied issues. It has become evident that, apart from some disorders caused by gene
            mutations, the majority of disorders originating from disturbances of rhythms arise
            from environmental influences and epigenetic changes. In this context, it was mandatory
            to think of and devise experiments on temporary scales, which exponentially increased
            the volumes of data obtained from time-series and rapidly became prohibitive of manual
            inspection. Therefore, more and more sophisticated mathematical algorithms have been
            developed to identify rhythmic expression of genes and to find coexpression by their
            clustering. It is expected that disturbed rhythmic behaviour in mental disorders is
            reflected in altered oscillatory behaviour of gene expression.
         
         Key words
circadian rhythms - sleep - mental disorders - microarrays - epigenetics - gap junctions
            - systems biology