Reverse electron transfer at mitochondrial complex I restrains dopaminergic neuron activity to promote early-life sleep in Drosophila

Sleep architecture and depth undergo profound changes across early life. In many species, including Drosophila melanogaster, juvenile animals exhibit elevated sleep drive and deeper sleep states relative to adults, a process linked to reduced activity of wake-promoting dopaminergic neurons (DANs). To identify cell-intrinsic mechanisms regulating developmental sleep, we profiled gene expression in juvenile and mature DANs and performed a targeted RNAi screen of genes with higher juvenile expression. From this screen, we found that the magnitude of mitochondrial complex I (MCI) disruption produced distinct behavioral outcomes. Severe MCI loss-of-function caused locomotor deficits due to mitochondrial dysfunction and reduced neuronal activity. Surprisingly, partial MCI inhibition preserved mitochondrial integrity but resulted in sleep loss, with a most pronounced impact on juvenile adult sleep fragmentation and depth. We demonstrate that dopaminergic neuron activity in juvenile flies is sensitive to the Coenzyme Q redox state with a low CoQ/CoQH2 promoting sleep depth by restraining DAN activity. Our results are consistent with a model in which the reverse transfer of electrons from CoQH2 to NAD+ at MCI limits DAN activity. By dissociating changes in CoQ redox state from catastrophic mitochondrial failure, this work indicates that sleep phenotypes may serve as sensitive indicators of emerging mitochondrial dysfunction, with implications for understanding the developmental origins of neurodegenerative vulnerability.