Endogenous rhythmic behaviors are evolutionarily conserved and essential for life. In mammalian and invertebrate models， well-characterized neuronal circuits and evolutionarily conserved mechanisms regulate circadian behavior and sleep [1-4]. In Drosophila， neuronal populations located in multiple brain regions mediate arousal， sleep drive， and homeostasis (reviewed in [3， 5-7]). Similar to mammals [8]， there is also evidence that fly glial cells modulate the neuronal circuits controlling rhythmic behaviors， including sleep [1]. Here， we describe a novel gene (CG14141; aka Nkt) that is required for normal sleep. NKT is a 162-amino-acid protein with a single IgC2 immunoglobulin (Ig) domain and a high-quality signal peptide [9]， and we show evidence that it is secreted， similar to its C.?elegans ortholog (OIG-4) [10]. We demonstrate that Nkt-null flies or those with selective knockdown in either neurons or glia have decreased and fragmented night sleep， indicative of a non-redundant requirement in both cell types. We show that Nkt is required in fly astrocytes and in a specific set of wake-promoting neurons-the mushroom body (MB) α'β' cells that link sleep to memory consolidation [11]. Importantly， Nkt gene expression is required in the adult nervous system for normal sleep， consistent with a physiological rather than developmental function for the Ig-domain protein.