Suprachiasmatic nucleus

The suprachiasmatic nucleus or nuclei (SCN) is a small region of the brain in the hypothalamus, situated directly above the optic chiasm. It is the principal circadian pacemaker in mammals, responsible for generating circadian rhythms. Reception of light inputs from photosensitive retinal ganglion cells allow it to coordinate the subordinate cellular clocks of the body and entrain to the environment. The neuronal and hormonal activities it generates regulate many different body functions in an approximately 24-hour cycle.

The idea that the SCN is the main circadian pacemaker in mammals was proposed by Robert Moore, who conducted experiments using radioactive amino acids to find where the termination of the retinohypothalamic projection occurs in rodents. Early lesioning experiments in mouse, guinea pig, cat, and opossum established how removal of the SCN results in ablation of circadian rhythm in mammals.

Moreover, the SCN interacts with many other regions of the brain. It contains several cell types and several different peptides (including vasopressin and vasoactive intestinal peptide) and neurotransmitters.

Disruptions or damage to the SCN has been associated with different mood disorders and sleep disorders, suggesting the significance of the SCN in regulating circadian timing