In this symposium, the circadian rhythms which coordinate human physiology at a fundamental level are analysed. The symposium is organized to take the audience on a journey from the molecular clock mechanism to the physiological “hands” of the human circadian clock, including what happens when circadian rhythms are disrupted, transiently during transmeridian travel, and chronically during shiftwork and in various diseases. First, Michael Hastings (MRC Laboratory of Molecular Biology, Cambridge) will describe the autoregulatory transcriptional/post-translational feedback loops and molecular components of the central circadian clock in the hypothalamus. The significance of peripheral clocks will also be discussed. Russell Foster (Brasenose College, Oxford) will then highlight the discovery of different photoreceptors and photopigment families, all of which appear to use a basic opsin/vitamin A-based photopigment biochemistry in order to synchronise the circadian clock to the light-dark cycle. Phil Ainslie (University of British Columbia, Okanagan, Canada) will then explore how the circadian clock manifests itself in cerebral haemodynamics highlighting the relevancy to diurnal variation in clinical syncope and strokes. Eus Van Someren (Netherlands Institute for Neuroscience, Amsterdam) will then examine how the functional relationships between circadian rhythms in core and peripheral body temperatures might be manipulated in order to improve sleep quality and vigilance in humans. Finally, Shawn Youngstedt (University of South Carolina, Columbia, USA) will examine how the effects of circadian rhythm disturbances might be ameliorated by carefully-timed exposure to the zeitgebers known to entrain the human circadian clock.