When light enters the eye through the pupil and is projected onto the retina, a pair of nuclei located directly above the optic chiasm, known as the suprachiasmatic nucleus (SCN), is activated. Situated in the hypothalamus of the brain, the SCN coordinates vertebrate metabolism and regulates the biological clock. It signals the pineal gland on when to secrete melatonin, thus adjusting the circadian rhythm.

During the day, light suppresses melatonin production. At this time, the endocrine system detects the lower concentration of melatonin and secretes hormones required to maintain alertness and focus during the day. As it gets dark and less light enters, melatonin production begins, signaling the body that it is nighttime.