The secretion of melatonin and the circadian rhythms are primarily is regulated by the suprachiasmatic nucleus (SCN) of the hypothalamus.
The action potentials from the SCN increases during the mid of the day 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.”
Thus, the blank can be filled with suprachiasmatic nucleus.