Project Summary Homeostasis is a fundamental physiological property, maintaining crucial state variables within a specific range for the viability and fitness of animals. Homeostatic mechanisms are also important for motivated behaviors, such as feeding or sleep. For example, prolonged wakefulness increases sleep drive (sleep pressure), leading to an increase in sleep amount and/or depth (sleep rebound). The cellular and molecular mechanisms underlying the homeostatic regulation of sleep remain unclear, and we have recently identified a novel neural circuit that encodes sleep drive. Although sleep drive is widely assumed to inevitably increase with greater wakefulness, exceptions to this rule exist in nature, particularly under conditions of high arousal. We hypothesize that specific signaling mechanisms act on this homeostatic circuit to suppress the accumulation and/or release of sleep drive. The overall goal of this proposal is to leverage these new findings to unravel the molecular and cellular mechanisms underlying sleep homeostasis. In addition, emerging data suggest an important role for glia in the homeostatic regulation of sleep. Thus, we propose to carry out the following aims: 1) characterize the signaling pathways acting on this novel circuit to regulate sleep drive; 2) characterize the circuit mechanisms acting downstream of this sleep homeostatic circuit to promote sleep; and 3) investigate a specific role for glia in the homeostatic regulation of sleep. To carry out these studies, we will use a variety of approaches, including behavioral assays, molecular genetic analyses, immunohistochemistry, functional imaging, and patch-clamp electrophysiology. These studies should reveal new insights into how sleep is homeostatically regulated. Insomnia and disorders of pathologic sleepiness are common and often associated with substantial morbidity. A better understanding of the mechanisms underlying sleep homeostasis may help in our search for novel treatments for both pathologic sleepiness and insomnia.