The regulatory architecture responsible for robust maintenance of 24 h cycles is analyzed as a control system. At the gene regulatory level, it is shown that performance attributes, notably phase timing, are controlled in a robust manner. At the next level in the hierarchy, it is shown that synchrony is achieved in populations of neurons to enable clock precision. Finally, at the level of the organism, it is shown that an optimal control approach can be used to reset the clock using a light stimulus. (c) 2006 Elsevier Ltd. All rights reserved.