Electrophysiological studies of neurons in the suprachiasmatic nucleus, of rat brain slices in vitro, reveal characteristic firing patterns which have been previously classified as regular, irregular and bursting. A new kind of neuronal firing is found, by means of an interspike interval distribution analysis. Based on these results a new classification scheme is suggested. A model to explain the presence of all the experimental patterns is delivered. The model not only suffices to infer the presence of regular oscillators, but also suggests interesting consequences concerning the components of the circadian system and their statistical behavior.