Our circadian clocks orchestrate endogenous biological rhythms in physiology, metabolism, and behavior that must be adjusted every day to maintain synchrony with the environment and produce optimally timed rhythmic outputs. This daily adjustment, known as entrainment, is achieved through a sensitivity of the brain's central clock neuron network to both external and internal time cues. Postindustrial society has significantly reduced the reliability of the cues that our circadian system uses for entrainment. This modern challenge to our clocks has wide-spread negative health consequences. The complexity of the brain's clock center is a significant barrier to a comprehensive understanding of the daily adjustment of the circadian system. Understanding the network properties of circadian timekeeping in the brain and the ways in which time cues impinge upon clock centers is critical if we are to address the significantly negative effects of circadian misalignment and dysfunction. The broad goal of this competing renewal application is a comprehensive understanding of the network properties of circadian timekeeping and entrainment in Drosophila melanogaster, whose circadian system features highly conserved molecular, anatomical, and physiological features that it shares with mammalian clock networks. Within this well-defined clock center, we will investigate how highly conserved external and internal time cues produce network-wide clock modulation and control sleep. The goals of our three specific aims are to elucidate: 1) the mechanistic basis of cholinergic and GABAergic inputs into the clock neuron network and the roles they play in sleep control and circadian timekeeping and entrainment, 2) the neurophysiological mechanisms by which the various clock neuron classes of the clock network modulate one another to produce coherent circadian rhythms, and 3) the anatomical and neurophysiological basis of the network integration of light and temperature cues from peripheral sensory receptors. The unifying goal of this proposal is to advance our understanding of circadian timekeeping and entrainment in the brain. The results of this work will ultimately inform the development and implementation of interventions designed to alleviate the significantly adverse metabolic and psychological effects of circadian dysfunction in the modern world.