Measuring seasonal time within the circadian system: regulation of the suprachiasmatic nuclei by photoperiod.

Day-length (photoperiod) is the primary environmental signal used to synchronise endogenous rhythms of physiology and behaviour. In mammals, the suprachiasmatic nuclei (SCN) of the hypothalamus house the master circadian clock. The SCN incorporate photoperiodic information and therefore measure both daily and seasonal time. Over the past decade, there have been significant advances in the understanding of the molecular basis of circadian clocks. It is now becoming apparent that the core molecular clock mechanism is itself regulated by photoperiod, although there is currently debate as to how this occurs. One recent model proposes that distinct groups of core 'clock genes' are associated with either morning or evening phases of the daily light/dark cycle. However, the validity of associating particular genes to morning and evening has been questioned. This article reviews the evidence for photoperiodic regulation of circadian clock function and then discusses alternative models that may explain the available data.