A Circuit Mechanism Underlying Suppression of Circadian Signals by Homeostatic Sleep Drive

Sleep is controlled by homeostatic mechanisms, which drive sleep following periods of wakefulness, and a circadian clock, which regulates sleep timing in a daily cycle. Homeostatic sleep drive sometimes overrides the clock, such that recovery sleep after deprivation occurs outside the normal circadian rest period. However, mechanisms underlying this effect are unknown. We find that sleep-promoting dorsal fan-shaped body (dFB) neurons, effectors of homeostatic sleep in Drosophila, are presynaptic to hugin+ neurons, previously identified as circadian output neurons regulating locomotor activity rhythms. Sleep deprivation decreases hugin+ neuronal activity, which likely suppresses circadian control to promote recovery sleep driven by dFB neurons. Indeed, removal of hugin+ neurons increases sleep-promoting effects of dFB neurons. Trans-synaptic mapping reveals that hugin+ neurons feed back onto s-LNv central clock neurons, which also show Hugin-dependent decreased activity upon sleep loss. These findings identify a circuit-based mechanism through which sleep drive modulates the circadian system to promote recovery sleep following deprivation.