Your internal biological clock — the ~24-hour cycle that regulates when you sleep, when hormones peak, when your immune system is most active, and dozens of other processes.The suprachiasmatic nucleus (SCN) in the hypothalamus serves as the master circadian pacemaker. VIP and PACAP are critical SCN synchronization signals — neuropeptides that help the internal clock stay aligned with the external light-dark cycle. Disruption of circadian rhythm is associated with metabolic dysfunction, immune dysregulation, and mood disorders, which is why the sleep and circadian peptide cluster is pharmacologically broader than just sleep aids.