The Drosophila core circadian circuit contains distinct groups of interacting neurons that give rise to diurnal sleep-wake patterns. Previous work showed that a subset of Dorsal Neurons 1 (DN1s) are sleep-promoting through their inhibition of activity-promoting circadian pacemakers. Here we show that these anterior-projecting DNs (APDNs) also \"exit\" the circadian circuitry and communicate with the homeostatic sleep center in higher brain regions to regulate sleep and sleep-wake arousal threshold. These APDNs connect to a small discrete subset of tubercular-bulbar neurons, which are connected in turn to specific sleep-centric Ellipsoid Body (EB)-Ring neurons of the central complex. Remarkably, activation of the APDNs produces sleep-like oscillations in the EB and also raises the arousal threshold, which requires neurotransmission throughout the circuit. The data indicate that this APDN-TuBusup-EB circuit temporally regulates sleep-wake arousal threshold in addition to the previously defined role of the TuBu-EB circuit in vision, navigation and attention.
