Circadian rhythms are a ubiquitous adaptation of eukaryotic organisms to the most reliable and predictable of environmental changes, the daily cycles of light and temperature. Prominent daily rhythms in behavior, physiology, hormone levels and biochemistry (including gene expression) are not merely responses to these environmental cycles, however, but embody the organism's ability to keep and tell time. At the core of circadian systems is a mysterious mechanism, located in the brain (actually the suprachiasmatic nucleus of the hypothalamus) of mammals, but present even in unicellular organisms, that functions as a clock. This clock drives circadian rhythms. It is independent of, but remains responsive to, environmental cycles (especially light). The interest in temporal regulation--its organization, mechanism and consequences--unites investigators in diverse disciplines studying otherwise disparate systems. This diversity is reflected in the brief reviews that summarize the presentations at a meeting on circadian rhythms held in New York City on October 31, 1992. The meeting was sponsored by the Fondation pour l'Etude du Systme Nerveux (FESN) and followed a larger meeting held 18 months earlier in Geneva, whose proceedings have been published (M. Zatz (Ed.), Report of the Ninth FESN Study Group on 'Circadian Rhythms', Discussions in Neuroscience, Vol. VIII, Nos. 2 + 3, Elsevier, Amsterdam, 1992). Some speakers described progress made in the interim, while others addressed aspects of the field not previously covered.
