Circadian rhythms in physiology and behaviour are orchestrated by the hypothalamic suprachiasmatic nucleus (SCN), the "master" clock of the body's circadian systems. Several neuropeptides, including vasopressin (VP), have been linked to the regulation of the circadian clock. The SCN contains a large population of VP neurons, but we recently found that the retina also contains many VP-expressing retinal ganglion cells (RGCs), and that, strikingly, these project exclusively to the SCN. VP signalling in the SCN determines the speed of re-entrainment the environmental clock after a phase advance in the light-dark cycle. Importantly, VP signalling seems to restrain re-entrainment. Generally, it has been assumed that the time-dependent ability of a light pulse to re-entrain circadian rhythms reflects the circadian state of the target neurons in the SCN. I will show that the VP-expressing RGCs constitute a major, light-activated excitatory pathway that has a key role in regulating circadian rhythms. Our current research is set to determine whether inhibition/activation of the pathway modulates circadian re-entrainment in response to light shifts. Improving circadian re-entrainment has the potential to positively impact millions of people whose working patterns involve struggling against the strongly encoded light-dark biological rhythms.