CLOCK and substance abuse: Primary areas of circadian-drug research in which Drosophila could provide advancement can be grouped into three strategic classifications: (1) system-level research (behavioral sensitivity and pathology) for defining the interactions between the circadian clock, sleep, and substance abuse; (2) identification of molecular networks for identifying the connections between the circadian system and substance neurobiology; and (3) drug discovery and small-molecule screening for therapy development.