Hugin + neurons provide a link between sleep homeostat and circadian clock neurons

Significance This manuscript identifies a mechanism by which sleep can sometimes occur at the wrong circadian time of day. Sleep is regulated by two processes, the circadian system, which confers 24-h rhythms on sleep, and the homeostatic system, which ensures an organism sleeps sufficiently. Sleep occurs at night because of circadian timing cues and also homeostatic sleep drive triggered by prolonged daytime wakefulness. However, the homeostatic system can sometimes override the circadian system; for instance, following a night of sleep deprivation. We identify a mechanism by which the homeostatic system suppresses circadian signals, allowing sleep to occur against the circadian clock. We find connections between homeostatic and circadian circuits and show that homeostatic sleep drive decreases circadian neuronal activity.