Sleep is an essential and evolutionarily conserved behavior that is closely related to synaptic function. However, whether neuroligins (Nlgs), which are cell adhesion molecules involved in synapse formation and synaptic transmission, are involved in sleep is not clear. Here, we show thatDrosophilaNlg4 (DNlg4) is highly expressed in large ventral lateral clock neurons (l-LNvs) and that l-LNv-derived DNlg4 is essential for sleep regulation. GABA transmission is impaired in mutant l-LNv, and sleep defects indnlg4mutant flies can be rescued by genetic manipulation of GABA transmission. Furthermore,dnlg4mutant flies exhibit a severe reduction in GABAAreceptor RDL clustering, and DNlg4 associates with RDLs invivo. These results demonstrate that DNlg4 regulates sleep through modulating GABA transmission in l-LNvs, which provides the first known link between a synaptic adhesion molecule and sleep inDrosophila.