The main questions in circadian neurobiology are: how many oscillators are involved; how are their daily oscillations generated and synchronized to the external world; and how do they signal time of day to the organism. The suprachiasmatic nuclei of the hypothalamus (SCN) are well established as the principal circadian oscillator of mammals. Their 10,000 or so 'clock' neurones drive our overt rhythms-the daily patterning we observe in our physiology and behaviour being mirrored perfectly by their spontaneous cycle of neuronal activity. However, they are not our only circadian oscillator, their molecular timekeeping is not understood and they ways in which they communicate with other parts of the brain are more unusual than was previously assumed.