הגרעין בהיפותלמוס האחראי על בקרת המחזור הצירקדיאני הינו:

The suprachiasmatic nucleus is tiny—less than 1 mm 3 and fewer than 10,000 neurons on each side—but it is the “master clock” for our circadian rhythms ( Fig. 23-5A ). The free-running period of cells in the suprachiasmatic nucleus is typically about 25 hours (see Fig. 23-5B ), but it receives direct projections from the retina that entrain it to the actual day length. Its neurons also contain numerous melatonin receptors, and the nighttime rise in pineal melatonin secretion is thought to provide an additional signal that helps “set” the circadian clock.