The action potentials from the suprachiasmatic nucleus (SCN) increases during the mid of the day (stimulated by light) and decreases by the mid of the night. The circadian rhythm is synchronized to the light-dark cycle by means of light, which act through the “retinohypothalamic tract.”
The light entrains the SCN shell cells, which activate slave oscillators by means of chemical signals and anatomical connections.
Thus, the blanks can be filled with shell; chemical; anatomical.