The central circadian clock of the suprachiasmatic nucleus (SCN) comprises a network of diverse neuronal and glial cell types, yet its operating mechanism remains elusive. Here, by monitoring cellular calcium rhythms in vivo using dual-color fiber photometry in mice lacking vasoactive intestinal polypeptide (VIP), we demonstrate that arginine vasopressin (AVP) neurons oscillate intrinsically with a short period and reduced amplitude. This indicates that VIP normally amplifies and phase-delays the AVP neuronal rhythm each day, thereby lengthening its period to approximately 24 h in constant darkness. Consistently, the behavioral circadian period is shortened by AVP neuron-specific VIP receptor dysfunction and lengthened by AVP neuron-specific blockade of neurotransmitter release. VIP neurons and other SCN cell types occasionally exhibit weak, unstable, long-period calcium rhythms only when the AVP neuronal oscillation is attenuated due to VIP deficiency or AVP neuron-specific Bmal1 deletion. Given that AVP neurons serve as the primary pacesetter cells of the SCN ensemble rhythm, these results indicate that an SCN neuronal feedback loop (SNFL), composed of the AVP cellular oscillator and VIPergic signaling, is essential for generating robust circadian rhythms.