The circadian clock as a molecular calendar.

There are two dominant environmental oscillators shaping the living conditions of our world: the day-night cycle and the succession of the seasons. Organisms have adapted to these by evolving internal clocks to anticipate these variations. An orchestra of finely tuned peripheral clocks slaved to the master pacemaker of the suprachiasmatic nuclei (SCN) synchronizes the body to the daily 24h cycle. However, this circadian clockwork closely interacts with the seasonal time-teller. Recent experiments indeed show that photoperiod--the dominant Zeitgeber of the circannual clock--might be deciphered by the organism using the tools of the circadian clock itself. From the SCN, the photoperiodic signal is transferred to the pineal where it is decoded as a varying secretion of melatonin. Different models have been proposed to explain the mechanism by which the circadian clock measures day-length. Recent work using mutant mice suggests a set of two molecular oscillators tracking dusk and dawn, respectively, thereby translating day-length to the body. However, not every aspect of photoperiodism is covered by this theory and majoradjustments will need tobe made toestablish a widely acceptable uniform model of circadian/circannual timekeeping.