Conserved circadian membrane rhythms arise from divergent cellular mechanisms in pacemaker neurons of mice and Drosophila

Circadian clocks align physiology and behaviour with daily environmental cycles, requiring neuronal networks that reliably encode and transmit time-of-day information. A fundamental yet unresolved question is how membrane properties support this function and whether conserved principles arise across evolutionarily distant organisms. We compared the intrinsic electrophysiological properties of identified circadian neurons from a nocturnal mammal (mouse, Mus musculus SCN VIP neurons) and a diurnal insect (fly, Drosophila melanogaster l-LNv PDF neurons) across the 24-h light-dark cycle. Despite [~]700 million years of evolutionary divergence and opposite temporal niches, we found neurons from both species exhibited similar rhythmic patterns in key parameters for encoding time-of-day, including resting membrane potential, spontaneous firing rate and cell capacitance, indicating convergent strategies for generating daily variation in excitability despite dramatically different network scales ([~]2200 VIPs vs. 8 l-LNvs). These shared outputs arose through distinct mechanisms: flies exhibited higher input resistance, greater excitability and relatively larger A-type potassium currents, while mice displayed larger sustained outward currents and post-inhibitory rebound excitation, absent in flies. Rheobase troughs occurred during the inactive phase in both species, while other parameters did not cycle. These differences likely shape how each neuronal type integrates salient inputs, consistent with their respective roles in processing sustained daytime light (flies) versus nocturnal cues (mice). Our findings reveal conserved functional goals but divergent electrophysiological strategies in circadian neurons, reflecting evolutionary adaptation to species-specific environmental demands. Such neurophysiological diversity highlights how evolution shapes cellular mechanisms to meet the requirements of temporal niche while preserving robust circadian timekeeping.