We use the fruit fly, Drosophila melanogaster, as a model to understand the molecular underpinnings of behavior and physiology. Like mammals, Drosophila have a sleep state that is controlled by circadian and homeostatic mechanisms. The homeostatic system drives the need to sleep while the circadian system ensures that sleep occurs with a ~24 hour rhythm. The molecular and cellular networks underlying these two types of regulation are distinct and yet they also intersect. Over the year, we and others have identified molecular components of the clock and also molecules required to transmit time-of-day signals form the clock to produce rhythmic sleep:wake. We have also identified genes that regulate sleep, and the cellular site of action of some of these genes. In recent work, we have been mapping the cellular circuits that give rise to rhythmic behavior. Cells that are part of the central clock have been well-studied, but those that receive signals from clock cells and transmit time-of-day signals to produce the overt behavioral rhythm are not known. Using a combination of genetic and anatomical approaches, we have identified some of these cells, and are addressing the connections between them. Together these studies are expected to lead to an understanding of how circadian and homeostatic regulation are integrated to produce sleep:wake cycles.