The suprachiasmatic nuclei (SCN) are two tiny regions of the brain situated in the hypothalamus, directly above the optic chiasm. They consist of clusters of a few thousand neurons, and are attributed to the control of circadian rhythms. Suprachiasmatic cells each autonomously generate a circadian rhythm, and the suprachiasmatic nuclei are synchronised by specialized light-sensitive cells in the retina (called intrinsically photoreceptive ganglion cells) which project through the optic nerve into the suprachiasmatic nuclei.

Axons from the SCN project into other areas of the brain, and SCN-cells also secrete chemicals which diffuse into the extracellular fluid of the brain. The suprachiasmatic nuclei are thought to be involved in the regulation of processes such as cycles of sleep and waking, core body temperature variation and hormone release, in correspondence with the time of day.