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 encode and transmit time-of-day information. Whether conserved electrophysiological principles underlie circadian output across evolution remains unclear. We compared the intrinsic electrophysiological properties of identified circadian neurons from a nocturnal mammal (mouse SCN VIP neurons) and a diurnal insect ( Drosophila l-LNv PDF neurons) across the 24-h light-dark cycle using harmonised electrophysiological recordings and matched analytical approaches. Despite ~700 million years of evolutionary divergence and opposite temporal niches, both neuron types exhibited similar rhythmic patterns in parameters, including resting membrane potential, spontaneous firing rate and cell capacitance, indicating a conserved pattern of daily excitability despite different network scales. These shared membrane properties arose through distinct mechanisms: flies exhibited higher input resistance, greater excitability and a greater relative contribution of A-type potassium currents, while mice displayed larger sustained outward currents and post-inhibitory rebound excitation, the latter absent in flies. Rheobase troughs occurred during the inactive phase in both species, while other parameters were stable. These differences likely shape how each neuronal type integrates inputs within its circadian circuits. Our findings reveal conserved functional outputs generated by divergent electrophysiological mechanisms, highlighting evolutionary flexibility in the membrane basis of circadian timekeeping.