During sleep, our biological clock prepares us for the forthcoming period of activity by controlling the release of hormones and the activity of the autonomic nervous system. Here, we review the history of the study of circadian rhythms and highlight recent observations indicating that the same mechanisms that govern our central clock might be at work in the cells of peripheral organs. Peripheral clocks are proposed to synchronize the activity of the organ, enhancing the functional message of the central clock. We speculate that peripheral visceral information is then fed back to the same brain areas that are directly controlled by the central clock. Both clock mechanisms are proposed to have a complementary function in the organization of behaviour and hormone secretion.