Interactions between photic and nonphotic stimuli to synchronize the master circadian clock in mammals

The master circadian clock is located in the suprachiasmatic nuclei (SCN) in mammals. The most powerful synchronizer of the SCN clock is the daily variation in light intensity. Several other nonphotic cues are well known to be able to shift or synchronize the circadian clock in the absence of photic cues. Some results obtained at systems, cellular and molecular levels provide evidence in contrast to the view that nonphotic signals reset the SCN clock independently of the mechanisms of photic synchronization. Rather, the SCN appear to integrate a wide range of information from the environment to fine-tune photic synchronization. The neuronal mechanisms underlying this integration are far from being understood. Nevertheless, in real-life situations, multiple interactions between photic and nonphotic cues could be of importance for the daily phase adjustment of the circadian clock and its control of the 24-h temporal organization of the whole organism.