Biological pacemakers dictate our daily schedules in physiology and behaviour. The molecules, cells and networks that underlie these circadian rhythms can now be monitored using long-term cellular imaging and electrophysiological tools, and initial studies have already suggested a theme--circadian clocks may be crucial for widespread changes in brain activity and plasticity. These daily changes can modify the amount or activity of available genes, transcripts, proteins, ions and other biologically active molecules, ultimately determining cellular properties such as excitability and connectivity. Recently discovered circadian molecules and cells provide preliminary insights into a network that adapts to predictable daily and seasonal changes while remaining robust in the face of other perturbations.