Circadian rhythms are inherent, ~24-hour cycles that orchestrate numerous physiological and behavioral processes in organisms ranging from cyanobacteria to humans. Disruption of these rhythms is strongly associated with metabolic imbalances and sleep disorders, underscoring their importance for overall health. In Drosophila melanogaster, the molecular clock is located in a small subset of neurons, yet it exerts widespread control over both peripheral tissues and diverse neural circuits. In the first part of this work, we investigate how the circadian clock impacts larval fat body metabolism. We find that wild-type fat body tissues exhibit clear ~24-hour cycles in gene expression and lipid homeostasis. Remarkably, in per0 (clock-deficient) mutants, we observe 12-hour ultradian oscillations in lipid peroxidation and reactive oxygen species, revealing a compensatory mechanism that partially maintains metabolic homeostasis when canonical 24-hour rhythms are absent. Next, we examine how circadian signals regulate sleep and neuronal excitability in the adult mushroom body (MB), a key center for learning, memory, and state-dependent behaviors. Our results show that clock neurons drive daily oscillations of Pka-C1 and Nf1 expression in MB neurons, leading to cAMP/PKA and Ca²⁺ rhythms that peak during the daytime and promote wakefulness. These rhythms are further modulated by dopaminergic pathways connecting the pacemaker neurons to specific MB subdomains, illustrating how circadian outputs shape neural network function and behavior. Together, these studies underscore the broad influence of the circadian clock on processes as distinct as metabolism and sleep. The discovery of alternative ultradian mechanisms in metabolically active tissues and the tight regulation of MB excitability by clock genes highlight how circadian rhythms safeguard organismal health and behavioral performance. Given the evolutionary conservation of clock components and circuitry, insights from Drosophila provide valuable perspectives on the regulation of physiological processes and behaviors and the consequences of their perturbation.