Circadian rhythms and photoperiodism.

The circadian timing system plays a critical role in the regulation of seasonal modifications in reproductive function. By detecting and transducing changes in the day-length (photoperiod), the neural substrates of the circadian system, including the supra-chiasmatic nuclei of the hypothalamus, trigger reproductive activity or quiescence at the appropriate seasons of the year in photoperiodic species. The circadian system also plays a role in the expression of endocrine changes that occur with seasonal breeding. Surges in luteinizing hormone secretion in female hamsters, for example, are either expressed daily during reproductive quiescence or suppressed on three out of the four days of the cycle during the breeding season. By such mechanisms a daily timer can be used in the regulation of cyclic events of much longer period.