CIRCADIAN-RHYTHMS

Circadian rhythms are a ubiquitous adaptation of eukaryotic organisms to the most reliable and predictable of environmental changes, the dai ly cycles of light and temperature. Prominent daily rhythms in behavio r, physiology, hormone levels and biochemistry (including gene express ion) are not merely responses to these environmental cycles, however, but embody the organism's ability to keep and tell time. At the core o f circadian systems is a mysterious mechanism, located in the brain (a ctually the suprachiasmatic nucleus of the hypothalamus) of mammals, b ut present even in unicellular organisms, that functions as a clock. T his clock drives circadian rhythms. It is independent of, but remains responsive to, environmental cycles (especially light). The interest i n temporal regulation - its organization, mechanism and consequences - unites investigators in diverse disciplines studying otherwise dispar ate systems. This diversity is reflected in the brief reviews that sum marize the presentations at a meeting on circadian rhythms held in New York City on October 31, 1992. The meeting was sponsored by the Fonda tion pour l'Etude du Systeme Nerveux (FESN) and followed a larger meet ing held 18 months earlier in Geneva, whose proceedings have been publ ished (M. Zatz (Ed.), Report of the Ninth FESN Study Group on 'Circadi an Rhythms', Discussions in Neuroscience, Vol. VIII, Nos. 2 + 3, Elsev ier, Amsterdam, 1992). Some speakers described progress made in the in terim, while others addressed aspects of the field not previously cove red.