Project Summary The goal of this proposal is to determine how the timing of sleep is controlled. Although the circadian clock drives the 24 hour rhythmicity of sleep, the onset and duration of sleep is also determined by the extent of prior wakefulness. The latter represents homeostatic control, which ensures that an organism gets enough sleep. In a daily cycle, circadian and homeostatic systems are aligned, so sleep occurs at night following a day of wakefulness. However, sleep loss at night or aspects of modern lifestyles, such as shiftwork or jetlag, can cause misalignment of circadian and homeostatic systems. For instance, following a night of sleep deprivation, homeostatic mechanisms will drive sleep in the morning although it is a time of circadian arousal. Even under these conditions, the circadian system influences sleep and so will curtail the amount of sleep a night shift worker might get during the day. Thus, interactions between circadian and homeostatic systems occur constantly and are poorly understood. Using a Drosophila model, we have dissected circuits by which the central brain clock controls rhythms of locomotor activity. We now find that specific circadian output neurons we identified make contact with loci that mediate homeostatic control of sleep. Our data suggest that the sleep homeostat suppresses circadian arousal signals to allow sleep at the wrong time of day, e.g. in the morning following a night of sleep deprivation. In addition, we find a rhythm of calcium in cells of the sleep homeostat as well as circadian regulation of their sleep output. Rhythmic sleep output could be due to parallel effects of specific clock neurons, which drive arousal at specific times of day, but together with the daily modulation of neural activity (calcium) in sleep homeostatic cells, these findings provide us with a handle to determine how circadian signals are integrated with homeostatic cues. To address the mechanisms by which these systems control the timing of sleep, we propose to: (1) Determine how and when cells of the sleep homeostat act on circadian output neurons to suppress arousal; (2) Determine how circadian and sleep signals are integrated in circadian output neurons. As these output neurons also feedback to clock neurons, they play a critical role in linking the circadian and homeostatic systems; (3) Determine how the clock interacts with light and homeostatic signals to modulate the output of sleep homeostatic neurons. Misaligned and disrupted sleep cycles have severe health consequences, including neurological disorders such as stroke, so we expect this work to have broad medical relevance.