suprachiasmatic nucleus

The suprachiasmatic nucleus, or SCN, is the body's master clock: a tiny cluster of neurons in the hypothalamus that sets the timing for the whole network of circadian rhythms. If every cell has its own little clock, the SCN is the conductor that keeps the entire orchestra playing in time. Its name comes from its position just above the optic chiasm where the optic nerves cross.

Sitting right above the visual pathway is no accident: the SCN receives a direct connection from the retina, so it learns the outside light-dark cycle and uses it to reset itself each day. It then broadcasts timing signals to the rest of the brain and body, including the pathway that controls the pineal gland's release of melatonin at night.

Through these outputs the SCN orchestrates the circadian patterns of cortisol, melatonin, body temperature, and the sleep-wake cycle. Damage to the SCN abolishes coherent daily rhythms, and the constant exposure to artificial light at night that defines modern life can distort the signals the SCN uses to stay aligned with the natural day.